ar71xx: add support for YunCore T830

ar71xx: add support for YunCore T830

YunCore T830 is a simple N300 router with 5-port FE switch, detachable
antennas and USB 2.0 port.

Specification:

- 650/597/216 MHz (CPU/DDR/AHB)
- 128 MB of RAM (DDR2)
- 16 MB of FLASH (SPI NOR)
- 5x 10/100 Mbps Ethernet
- 2T2R 2.4 GHz (QCA9531), with ext. PA (SKY65174-21) and LNA
- two external, detachable antennas (RP-SMA)
- 1x USB 2.0
- 8x LED (7 driven by GPIO)
- 1x button (reset)
- DC jack for main power input (12 V)
- UART and JTAG headers on PCB

Flash instruction:

1. First, gain root access to the device, following below steps:

- Login into web gui (default password/IP: admin/192.168.188.253).
- Go to "Advanced" -> "Management" -> "System" and download backup of
  configuration (bakfile.bin).
- Open the file as tar.gz archive, edit/update "shadow" file and change
  hash of root password to something known.
- Repack the archive, rename it back to "bakfile.bin" and use to
  restore configuration of the device.
- After that, device will reboot and can be accessed over SSH.

2. Then, install OpenWrt:

- Login over SSH and issue command:
  fw_setenv bootcmd "bootm 0x9f050000 || bootm 0x9fe80000"
- Upload "sysupgrade" image and install it (only if previous command
  succeeded) with command: "sysupgrade -n -F openwrt-...".

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>