ath79: add support for Netgear WNDR3700v4

ath79: add support for Netgear WNDR3700v4

This patch adds ath79 support for Netgear WNDR3700v4.
Router was previously supported by ar71xx target only.
Note: device requires 'ar934x-nand' driver in kernel.

Specification
=============
  * Description: Netgear WNDR3700v4
  * Loader: U-boot
  * SOC: Atheros AR9344 (560 MHz)
  * RAM: 128 MiB
  * Flash: 128 MiB (NAND)
    - U-boot binary: 256 KiB
    - U-boot environment: 256 KiB
    - ART: 256 KiB
    - POT: 512 KiB
    - Language: 2 MiB
    - Config: 512 KiB
    - Traffic Meter: 3 MiB
    - Firmware: 25 MiB
    - ART Backup: 256 KiB
    - Reserved: 96 MiB
  * Ethernet: 5 x 10/100/1000 (4 x LAN, 1 x WAN) (AR8327)
  * Wireless:
    - 2.4 GHz b/g/n (internal)
    - 5 GHz a/n (AR9580)
  * USB: yes, 1 x USB 2.0
  * Buttons:
    - Reset
    - WiFi (rfkill)
    - WPS
  * LEDs:
    - Power (amber/green)
    - WAN (amber/green)
    - WLAN 2G (green)
    - WLAN 5G (blue)
    - 4 x LAN (amber/green)
    - USB (green)
    - WPS (amber/green)
  * UART: 4-pin connector JP1, 3.3V (Vcc, TX, RX, GND), 115200 8N1
  * Power supply: DC 12V 2.5A
  * MAC addresses: LAN=WLAN2G on case label, WAN +1, WLAN5G +2

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

Note about partitioning: firmware partition offset (0x6c0000) is
hardcoded into vendor's u-boot, so this partition cannot be moved
and resized to include Netgear-specific flash areas (pot, language,
config, traffic_meter) not used by OpenWrt.

Test build configuration
========================
CONFIG_TARGET_ath79=y
CONFIG_TARGET_ath79_nand=y
CONFIG_TARGET_ath79_nand_DEVICE_netgear_wndr3700-v4=y
CONFIG_ALL_KMODS=y
CONFIG_DEVEL=y
CONFIG_CCACHE=y
CONFIG_COLLECT_KERNEL_DEBUG=y
CONFIG_IMAGEOPT=y

Signed-off-by: Paul Blazejowski <paulb@blazebox.homeip.net>