ramips: add support for Asus RT-N11P / RT-N12+ / RT-N12E b1

ramips: add support for Asus RT-N11P / RT-N12+ / RT-N12E b1

This is a variant of the MT7620N-based Asus routers.

Specifications:

- MT7620N (580 MHz)
- 32 MB RAM
- 8 MB Flash
- 5x 10/100Mbps Ethernet (built-in switch)
- 2.4 GHz WLAN
- 2x external, non-detachable antennas
- UART (J2) header on PCB (115200 8n1)

Flash instructions:

1. Configure PC with static IP 192.168.1.75/24
2. Connect PC with one of LAN ports, press the reset button, power up
   the router and keep button pressed for around 6-7 seconds. All 4 LEDs will
   start to blink, which is when the router will accept firmware files via TFTP.
   No known limitations on firmware filenames, just send it with a TFTP client
   to 192.168.1.1.
3. Router will download file from server, write it to flash and reboot.

Signed-off-by: Zoltan HERPAI <wigyori@uid0.hu>