ath79: add support for AVM FRITZ!WLAN Repeater 1750E

ath79: add support for AVM FRITZ!WLAN Repeater 1750E

This commit adds support for the AVM Fritz!WLAN Repeater 1750E

SOC:	Qualcomm QCA9556 (Scorpion) 720MHz MIPS74Kc
RAM:    64MB Zentel A3R12E40CBF DDR2
FLASH:  16MiB Winbond W25Q128 SPI NOR
WLAN1:  QCA9556 2.4 GHz 802.11b/g/n 3x3
WLAN2:  QCA9880 5 GHz 802.11 n/ac 3x3
INPUT:  WPS button
LED:    Power, WiFi, LAN, RSSI indicator
Serial: Header Next to Black metal shield
        Pinout is 3.3V - RX - TX - GND (Square Pad is 3.3V)
        The Serial setting is 115200-8-N-1.

Tested and working:
 - Ethernet
 - 2.4 GHz WiFi (correct MAC)
 - 5 GHz WiFi (correct MAC)
 - Installation via EVA bootloader
 - OpenWRT sysupgrade
 - Buttons
 - LEDs

Installation via EVA:
In the first seconds after Power is connected, the bootloader will
listen for FTP connections on 192.168.178.1. Firmware can be uploaded
like following:

  ftp> quote USER adam2
  ftp> quote PASS adam2
  ftp> binary
  ftp> debug
  ftp> passive
  ftp> quote MEDIA FLSH
  ftp> put openwrt-sysupgrade.bin mtd1

Note that this procedure might take up to two minutes.
You need to powercycle the Device afterwards to boot OpenWRT.

Signed-off-by: David Bauer <mail@david-bauer.net>