ath79: add support for Netgear WNR2200

ath79: add support for Netgear WNR2200

This patch adds ath79 support for Netgear WNR2200.
Router was previously supported by ar71xx target only (8 MiB variant).
Netgear WNR2200 has two flash versions - 8MiB sold in EU, US etc. and
16 MiB for Russia and China markets. Apart from flash size both variants
share the same hardware specification.

Specification
=============
  * Description: Netgear WNR2200
  * Loader: U-boot
  * SOC: Atheros AR7241 (360 MHz)
  * RAM: 64 MiB
  * Flash: 8 MiB or 16 MiB (SPI NOR)
	- U-boot binary: 256 KiB
	- U-boot environment: 64 KiB
	- Firmware: 7808 KiB or 16000 KiB
	- ART: 64 KiB
  * Ethernet: 4 x 10/100 LAN + 1 x 10/100 WAN
  * Wireless: 2.4 GHz b/g/n (Atheros AR9287)
  * USB: yes, 1 x USB 2.0
  * Buttons:
	- Reset
	- WiFi (rfkill)
	- WPS
  * LEDs:
	- Power (amber/green)
	- WAN (amber/green)
	- WLAN (blue)
	- 4 x LAN (amber/green)
	- WPS (green)
  * UART: 4-pin connector JP1, 3.3V (Vcc, TX, RX, GND), 115200 8N1
  * Power supply: DC 12V 1.5A
  * MAC addresses: LAN on case label, WAN +1, WLAN +2

Installation
============
  * TFTP recovery
  * TFTP via U-boot prompt
  * sysupgrade
  * Web interface

Test build configuration
========================
CONFIG_TARGET_ath79=y
CONFIG_TARGET_ath79_generic=y
CONFIG_TARGET_ath79_generic_DEVICE_netgear_wnr2200-8m=y
CONFIG_ALL_KMODS=y
CONFIG_DEVEL=y
CONFIG_CCACHE=y
CONFIG_COLLECT_KERNEL_DEBUG=y
CONFIG_IMAGEOPT=y
CONFIG_KERNEL_DEBUG_INFO=y
CONFIG_KERNEL_DEBUG_KERNEL=y

Signed-off-by: Michal Cieslakiewicz <michal.cieslakiewicz@wp.pl>