ath79: add support for COMFAST CF-E355AC v2

ath79: add support for COMFAST CF-E355AC v2

COMFAST CF-E355AC v2 is a ceiling mount AP with PoE support,
based on Qualcomm/Atheros QCA9531 + QCA9886.

Short specification:

- 1x 10/100 Mbps Ethernet, with PoE support (wan/eth1)
- 1x 10/100/1000 Mbps Ethernet, with PoE support (lan/eth0)
- 128MB of RAM (DDR2)
- 16 MB of FLASH
- 2T2R 2.4 GHz, 802.11b/g/n (wlan2g)
- 2T2R 5 GHz, 802.11ac/n/a, WAVE 2 (wlan5g)
- built-in 4x 3 dBi antennas
- output power (max): 500 mW (27 dBm)
- 1x RGB LED, 1x button
- separate watchdog chip via GPIO (bottom of PCB?)
- UART header on PCB with proper labelling

Markings on PCB:

* R121QH_VER2.1 (silkscreen, bottom)
* CF-WA800 (sticker, top)

Initial flashing instructions:

Original firmware is based on OpenWrt.

a) Use sysupgrade image directly in vendor GUI.

b) Or via tftp:

  ipaddr=192.168.1.1
  serverip=192.168.1.10
  bootfile="firmware.bin"

c) Or possibly via u-boot's `httpd` command.

MAC-address mapping follows original firmware:

* eth1 (wan) is the lowest mac address (art @ 0x0)
* eth0 (lan) uses eth1 + 1 (art @ 0x1002)
* wlan2g (phy1) uses eth1 + 2 (art @ 0x06)
* wlan5g (phy0) uses eth1 + 10 (not present in art)
* unused MAC (eth1 + 3) (art @ 0x5006)

Art dump (`hexdump /dev/mtd1 |grep ZZZZ`):

  0000000 ZZZZ XXXX XXX0 ZZZZ XXXX XXX2 ffff ffff
  0001000 0202 ZZZZ XXXX XXX1 0000 0000 0000 0000
  0005000 202f bd21 0101 ZZZZ XXXX XXX3 0000 2000

Root access to original firmware (only via UART) can be achieved by
making a backup of configuration from web interface. Backup contains
whole `/etc` directory...

Signed-off-by: Priit Laes <plaes@plaes.org>
Link: https://github.com/openwrt/openwrt/pull/16556
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>