ath79: add support for Comfast CF-EW72

ath79: add support for Comfast CF-EW72

Specifications:
Qualcomm/Atheros QCA9531 + QCA9886
2x 10/100 Mbps Ethernet, with 48v PoE
2T2R 2.4 GHz, 802.11b/g/n
2T2R 5 GHz, 802.11a/n/ac
128MB RAM
16MB SPI Flash
4x LED (Always On Power, LAN, WAN, WLAN)

Flashing Instructions:
Original firmware is based on OpenWRT, so flashing the sysupgrade image on
the factory firmware is sufficient.

Tested: Reset button, WAN LED, LAN LED, Power LED (always on, not much
to test), WLAN LED (one LED only for 2 interfaces, by default it gets
assigned to the first interface), MAC addresses (match factory firmware).
My LAN factory MAC address ends in F2.

use	stock_mac	art_loc
lan	:f2		0x0
wan	:f3		0x1002
5g	:f4		0x6
2g	:f5		0x5006

Since MAC address flash locations do not really match their use in vendor
firmware (e.g. address from 5 GHz calibration data is assigned to 2.4 GHz
WiFi), just calculate the MAC addresses with an offset based on 0x0 address.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
[add MAC address comment]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>