ath79: add TP-Link TL-WR710N v1

ath79: add TP-Link TL-WR710N v1

This commit adds support for TP-Link TL-WR710N v1 router.

CPU: Atheros AR9331 400MHz
RAM: 32MB
FLASH: 8MiB
PORTS: 1 Port 100/10 LAN (connected to a switch), 1 Port 100/10 WAN
WiFi: Atheros AR9331 1x2:1 bgn
USB: ChipIdea HDRC USB2.0
LED: SYS
BTN: Reset

Sysupgrade from `ar71xx` works without glitches.
Network interfaces assigned for LAN and WAN ports are `eth1` and `eth0`
respectively, what's consistent with `ar71xx` target. Wireless radio
path is automatically upgraded from `platform/ar933x_wmac` to
`platform/ahb/18100000.wmac`.

Signed-off-by: Marcin Jurkowski <marcin1j@gmail.com>