ath79: add support for Comfast CF-EW71 v2

ath79: add support for Comfast CF-EW71 v2

Specifications:
Qualcomm/Atheros QCA9531
2x 10/100 Mbps Ethernet, with 48v PoE
2T2R 2.4 GHz, 802.11b/g/n
128MB RAM
16MB SPI Flash
4x LED (Always On Power, LAN, WAN, WLAN)

Flashing instructions:
The original firmware is based on OpenWrt, so flashing the sysupgrade image over the factory firmware is sufficient.
The bootloader has a built-in recovery web-ui. This is the method I used to flash OpenWrt. You can get to the recovery web-ui by holding down the reset button for a few seconds (~5s) while pluggin in the router. The LEDs should start blinking fast and the router should be available on 192.168.1.1 for the recovery.

Tested: Reset button, WAN LED, LAN LED, Power LED (always on, not much to test), WLAN LED, MAC addresses (same as factory firmware).

Signed-off-by: Felix Golatofski <git@xdfr.de>