ramips: add support for TP-Link TL-WR902AC v3

ramips: add support for TP-Link TL-WR902AC v3

TP-Link TL-WR902AC v3 is a pocket-size dual-band (AC750) router
based on MediaTek MT7628N + MT7650E.

Specification:
- MediaTek MT7628N/N (580 Mhz)
- 64 MB of RAM
- 8 MB of FLASH
- 2T2R 2.4 GHz and 1T1R 5 GHz
- 1x 10/100 Mbps Ethernet

* MT7650 ac chip isn't not supported by LEDE/OpenWrt at the moment.
  Therefore 5Ghz won' work.

Flash instruction:

The only way to flash LEDE image in TL-WR902AC v3 is to use
tftp recovery mode in U-Boot:

1. Configure PC with static IP 192.168.0.66/24 and tftp server.
2. Rename "openwrt-ramips-mt76x8-tplink_tl-wr902ac-v3-squashfs-tftp-recovery.bin"
   to "tp_recovery.bin" and place it in tftp server directory.
3. Connect PC with the LAN port, press the reset button, power up
   the router and keep button pressed for around 6-7 seconds, until
   device starts downloading the file.
4. Router will download file from server, write it to flash and reboot.

Signed-off-by: Peter Lundkvist <peter.lundkvist@gmail.com>
[drop p2led_an pinmux, this pin isn't used as gpio, fix whitespace issues]
Signed-off-by: Mathias Kresin <dev@kresin.me>