Edit: Didn't find a solution to this, but installing Manjaro without a realtime driver solved the problem 
Hi, I am having a problem with Wifi on my new install of Manjaro on my early 2015 Macbook Pro. (A1502 EMC 2835, a 13" retina model) Previously I had Antergos installed and the wifi worked fine on that, but I switched to Manjaro because I heard Antergos was being discontinued.
iwconfig shows this:
lo no wireless extensions.
enp0s20u1u4 no wireless extensions.
The second one is the usb ethernet adaptor I bought to be able to connect to the internet, but obviously I don't want to need an ethernet port or android tether to use internet on a laptop.
journalctl | grep brcmfmac shows these messages, I think every time the OS tries to start up the BCM43602. Please excuse the fact that my computer is called imgay, it's an in joke (I'm a lesbian).
Oct 06 21:02:06 imgay kernel: usbcore: registered new interface driver brcmfmac
Oct 06 21:02:06 imgay kernel: brcmfmac: brcmf_fw_alloc_request: using brcm/brcmfmac43602-pcie for chip BCM43602/1
Oct 06 21:02:06 imgay kernel: brcmfmac 0000:03:00.0: Direct firmware load for brcm/brcmfmac43602-pcie.Apple Inc.-MacBookPro12,1.txt failed with error -2
Oct 06 21:02:06 imgay kernel: brcmfmac 0000:03:00.0: Direct firmware load for brcm/brcmfmac43602-pcie.txt failed with error -2
Oct 06 21:02:07 imgay kernel: brcmf_fw_complete_request+0x3e9/0xa10 [brcmfmac]
Oct 06 21:02:07 imgay kernel: brcmf_fw_request_done+0x3d/0xe0 [brcmfmac]
Oct 06 21:02:07 imgay kernel: Modules linked in: brcmfmac brcmutil mmc_core isofs cdc_ether usbnet r8152 mii input_leds bnep hid_apple joydev mousedev hid_generic btusb btrtl btbcm btintel bluetooth ecdh_generic usbhid ecc bcm5974 hid uas usb_storage fuse nf_tables_set nft_fib_inet nft_fib_ipv4 nft_fib_ipv6 nft_fib nft_reject_inet nf_reject_ipv4 nf_reject_ipv6 nft_reject nft_ct nft_chain_nat nf_tables nfnetlink ip6table_nat ip6table_mangle ip6table_raw ip6table_security iptable_nat nf_nat iptable_mangle iptable_raw iptable_security nf_conntrack nf_defrag_ipv6 nf_defrag_ipv4 libcrc32c ip6table_filter ip6_tables iptable_filter 8021q garp mrp stp llc nls_iso8859_1 nls_cp437 vfat fat intel_rapl ofpart cmdlinepart intel_spi_platform x86_pkg_temp_thermal mei_hdcp intel_spi intel_powerclamp spi_nor mtd iTCO_wdt coretemp iTCO_vendor_support kvm_intel kvm irqbypass crct10dif_pclmul snd_hda_codec_hdmi crc32_pclmul ghash_clmulni_intel aesni_intel aes_x86_64 crypto_simd cryptd glue_helper applesmc input_polldev
Another possibly relevant thing, if I do sudo modprobe brcm80211 it says this:
modprobe: FATAL: Module brcm80211 not found in directory /lib/modules/4.19.59-rt23-MANJARO
I don't know where to get the module or if that would even fix the problem.
Things I have tried:
I downgraded kernel from linux52-rt to linux419-rt to see if those drivers would work, but the same errors occur. Should I move back to linux52-rt? I'm using the rt kernels because I want to do low-latency audio stuff, should I try a non rt kernel?
I've tried installing broadcom-wl, broadcom-wl DKMS version, but those didn't work.
I've checked the Arch Wiki entry on Mac, but the issues mentioned there didn't apply for me.
I've checked lots of forums where people have similar problems, but the posts I've read about similar issues with wifi not working on linux macbooks seem to have different root causes.
Also, for some reason the macbook has a looot of lag when booting up. In the macOS recovery screen for example, the planet "spins" smoothly, but the input lags and only intermittently moves the cursor or registers keyboard input. This happens in grub as well, it happens right up until I've successfully logged into manjaro. This topic isn't for advice about how to fix that, I only added the info because I thought it could possibly be relevant.
I would appreciate any guidance on how to fix this issue 
