ath79: add support for WD My Net Wi-Fi Range Extender

ath79: add support for WD My Net Wi-Fi Range Extender

This patch ports over support for the device from ar71xx.

SOC:	AR7370 (Wasp - AR9344 rev2 0001974c)
RAM:	Winbond W9725G6KB-25 32MiB
FLASH:	Winbond 25Q64FVSIG 8MiB
WLAN:	AR9380 Dual-Band 802.11abgn 3x3:3
INPUT:	WPS, RESET button (hardware on/off toggle button)
LED:    Power, LAN, WiFi, 3 RSSI-Leds (low, medium, high)
Serial: Header Next to the winbond flash chip (labeld JP1)
	Pinout is GND - NC - RX - TX - 3V3 (JP1)
	The Serial setting is 115200-8-N-1.

- Installation via uboot's upgrade command
  0. attach serial cable
  1. interrupt uboot and enter "upgrade code.bin" into
      the u-boot prompt
	ar7240> upgrade code.bin

  2. rename openwrt...sysupgrade.bin to code.bin on PC
  3. run a tftp-client on the PC
	 (shell)$ tftp 192.168.1.230
	 binary
	 put code.bin
  4. wait for the device to finish

	[...]
	Copy buff to Flash from 0x9f040000 length 0x79f000
	Copy to Flash... write addr: 9f040000
	done

  5.  enter "go" in the u-boot prompt
	ar7240> go

- TFTP ramdisk image boot from the uboot prompt
  (tftp server defaults to serverip 192.168.1.254)
	=> tftpboot 81000000 initramfs.bin
	=> bootm

Tested and working:
	- LEDs
	- Buttons
	- Ethernet
	- Wi-Fi
	- OpenWRT sysupgrade

For flashing and debricking information see:
<https://openwrt.org/toh/wd/rext>

Users coming from ar71xx can use sysupgrade too. But I highly
advise to no save the old configuration and start from a clean
state.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>