ar71xx: add support for the Airtight C-60

This patch adds support for the Airtight C-60.

SOC:	Atheros AR9344 rev 2 (CPU:560.000MHz)
RAM:	128 MiB
NOR:	MX25L3205D 4MiB
NAND:	ST Micro NAND 32MiB 3,3V 8-bit
SW-NET:	AR8327N (2 Ports)
WLAN1:	Dual-Band AR9340 Rev:2 (built-in SoC)
WLAN2:	Dual-Band AR9300 Rev:4 PCIe Chip

The switch is setup for an accesspoint:
	LAN1: (gigabit) is the wan-port.
	LAN2: (fast ethernet) is bridged with the br-lan.

Flashing Guide (via initramfs):
  1. Connect a PC to the serial port of the C-60.
     power up the C-60.

     Enter u-boot command prompt:
     #> nand erase
     #> setenv bootcmd "bootm 0x9f060000"
     #> saveenv
     #> setenv ipaddr 192.168.1.1
     #> setenv netmask 255.255.255.0
     #> setenv serverip 192.168.1.100
     #> setenv bootfile lede-ar71xx-nand-c-60-initramfs-kernel.bin
     #> tftpboot
     #> bootm

  2. Wait for the C-60 to boot LEDE.
     On the root prompt. Enter:
     # ubiformat /dev/mtd4
     # ubiattach -p /dev/mtd4

  3. After that copy the sysupgrade.tar onto the router and run:
     # sysupgrade sysupgrade.tar

     to flash the image.

Special thanks to Chris Blake <chrisrblake93@gmail.com>. He provided
a C-60 unit and he helped with debugging the switch, LEDs and platfrom
support.

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