ath79: add support for Sophos AP15

ath79: add support for Sophos AP15

The Sophos AP15 seems to be very close to Sophos AP55/AP100.

Based on:
commit 6f1efb289837 ("ath79: add support for Sophos AP100/AP55 family")
author    Andrew Powers-Holmes <andrew@omnom.net>
          Fri, 3 Sep 2021 15:53:57 +0200 (23:53 +1000)
committer Hauke Mehrtens <hauke@hauke-m.de>
          Sat, 16 Apr 2022 16:59:29 +0200 (16:59 +0200)

Unique to AP15:
 - Green and yellow LED
 - 2T2R 2.4GHz 802.11b/g/n via SoC WMAC
 - No buttons
 - No piezo beeper
 - No 5.8GHz

Flashing instructions:
 - Derived from UART method described in referenced commit, methods
   described there should work too.
 - Set up a TFTP server; IP address has to be 192.168.99.8/24
 - Copy the firmware (initramfs-kernel) to your TFTP server directory
   renaming it to e.g. boot.bin
 - Open AP's enclosure and locate UART header (there is a video online)
 - Terminal connection parameters are 115200 8/N/1
 - Connect TFTP server and AP via ethernet
 - Power up AP and cancel autoboot when prompted
 - Prompt shows 'ath> '
 - Commands used to boot:
    ath> tftpboot 0x81000000 boot.bin
    ath> bootm 0x81000000
 - Device should boot OpenWRT
 - IP address after boot is 192.168.1.1/24
 - Connect to device via browser
 - Permanently flash using the web ui (flashing sysupgrade image)
 - (BTW: the AP55 images seem to work too, only LEDs are not working)

Testing done:
 - To be honest: Currently not so much testing done.
 - Flashed onto two devices
 - Devices are booting
 - MAC addresses are correct
 - LEDs are working
 - Scanning for WLANs is working

Big thanks to all the people working on this great project!
(Sorry about my english, it is not my native language)

Signed-off-by: Manuel Niekamp <m.niekamp@richter-leiterplatten.de>