bcm27xx: add support for Raspberry Pi Zero 2

Due to licensing uncertainty, we do not include the firmwares for the
wireless chips used in the Raspberry Pi Zero 2 W. To have working
wireless, follow the instructions below.

For people building their own images:

  mkdir -p files/lib/firmware/brcm
  wget -P files/lib/firmware/brcm/ https://github.com/RPi-Distro/firmware-nonfree/raw/bullseye/debian/config/brcm80211/brcm/brcmfmac43436-sdio.bin
  wget -P files/lib/firmware/brcm/ https://github.com/RPi-Distro/firmware-nonfree/raw/bullseye/debian/config/brcm80211/brcm/brcmfmac43436-sdio.txt
  wget -P files/lib/firmware/brcm/ https://github.com/RPi-Distro/firmware-nonfree/raw/bullseye/debian/config/brcm80211/brcm/brcmfmac43436s-sdio.bin
  wget -P files/lib/firmware/brcm/ https://github.com/RPi-Distro/firmware-nonfree/raw/bullseye/debian/config/brcm80211/brcm/brcmfmac43436s-sdio.txt

Now build the OpenWrt image as usual, and it will include the firmware
files in the correct location.

For people using ext4 images:

Write the ext4 image to the sdcard, then mount the 2nd partition and put
the firmware files from the links above in /lib/firmware/brcm relative
from the mount point where the partition is mounted.

For people using squashfs images:

Write the squashfs image to the sdcard, place it in the Raspberry Pi
Zero 2 W, boot it and wait for the overlay filesystem to be created.
Find the offset of the overlay filesystem in sysfs:

  # cat /sys/devices/virtual/block/loop0/loop/offset
  25755648

Shut down the device, unplug the power and move the SD card to a Linux
computer. Mount the 2nd partition of the sdcard as a loop device with
the offset found earlier.

  sudo mount /dev/sdh2 -o loop,offset=25755648 /mnt/temp

Put the firmware files from the links above in /upper/lib/firmware/brcm
relative to the mount point where the loop device is mounted.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Tested-by: Peter van Dijk <peter@7bits.nl>

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