ar71xx: add support for Fritz!Box 4020

ar71xx: add support for Fritz!Box 4020

This commit adds support for the AVM Fritz!Box 4020 WiFi-router.

SoC:   Qualcomm Atheros QCA9561 (Dragonfly) 750MHz
RAM:   Winbond W971GG6KB-25
FLASH: Macronix MX25L12835F
WiFi:  QCA9561 b/g/n 3x3 450Mbit/s
USB:   1x USB 2.0
IN:    WPS button, WiFi button
OUT:   Power LED green, Internet LED green, WLAN LED green,
       LAN LED green, INFO LED green, INFO LED red
UART:  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 (LAN + WAN)
 - WiFi (correct MAC)
 - Installation via EVA bootloader
 - OpenWRT sysupgrade
 - Buttons
 - LEDs

Not working:
 - USB port

Installation via EVA:
In the first seconds after Power is connected, the bootloader will
listen for FTP connections on 169.254.157.1 (Might also be 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. After transfer is
complete you need to powercycle the device to boot OpenWRT.

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