ar71xx:add support for COMFAST CF-E375AC

ar71xx:add support for COMFAST CF-E375AC

COMFAST CF-E375AC is a ceiling mount AP with PoE support, based on
Qualcomm/Atheros QCA9563 + QCA9886 + QCA8337.

Short specification:

    2x 10/100/1000 Mbps Ethernet, with PoE support
    128MB of RAM (DDR2)
    16 MB of FLASH
    3T3R 2.4 GHz, 802.11b/g/n
    2T2R 5 GHz, 802.11ac/n/a, wave 2
    built-in 5x 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>