ar71xx: add support for Fritz!WLAN Repeater 450E

ar71xx: add support for Fritz!WLAN Repeater 450E

This commit adds support for the AVM Fritz!WLAN Repeater 450E

SOC:    Qualcomm QCA9556 (Scorpion) 560MHz MIPS74Kc
RAM:    64MB Zentel A3R12E40CBF DDR2
FLASH:  16MiB Winbond W25Q128 SPI NOR
WLAN1:  QCA9556 2.4 GHz 802.11b/g/n 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
 - 2.4GHz WiFi (correct MAC)
 - Installation via EVA bootloader
 - OpenWRT sysupgrade
 - Buttons
 - Most LEDs

Not working:
 - 2 RSSI LEDs
   AVM used for RSSI{0,1} two of the Ethernet PHYs LEDs which they
   control over MDIO. Our driver doesn't expose these LEDs as GPIOs.
   While it is possible to implement this feature, it would require an
   additional kernel patch for a minor functionality.

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>