ar71xx: add support for COMFAST CF-E355AC v2

ar71xx: 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:

- 2x 10/100 Mbps Ethernet, with PoE support
- 128MB of RAM (DDR2)
- 16 MB of FLASH
- 2T2R 2.4 GHz, 802.11b/g/n
- 2T2R 5 GHz, 802.11ac/n/a, WAVE 2
- built-in 4x 3 dBi antennas
- output power (max): 500 mW (27 dBm)
- 1x RGB LED, 1x button
- built-in watchdog chipset

Flash instruction:

Original firmware is based on OpenWrt.
Use sysupgrade image directly in vendor GUI.

Signed-off-by: Ding Tengfei <dtf@comfast.cn>
[updated kernel config for both boards]
Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>