ramips: add support for Asus RT-AC51U

ramips: add support for Asus RT-AC51U

Specification:
 - SoC: MediaTek MT7620A (580 MHz)
 - RAM: 64 MiB (Winbond W9751G6JB-25)
 - Flash: 16 MiB (Spansion S25FL128SAIF00)
 - LAN: x4 100M
 - WAN: x1 100M
 - Others: USB 2.0, reset button, wps button and 9 LEDs

Issues:
 - 5 GHz band is not functional (missing driver support)

Installation:

Asus windows recovery tool:
 - install the Asus firmware restoration utility
 - unplug the router, hold the reset button while powering it on
 - release when the power LED flashes slowly
 - specify a static IP on your computer:
     IP address: 192.168.1.75;
     Subnet mask 255.255.255.0
 - Start the Asus firmware restoration utility, specify the sysupgrade
   image, and press upload

TFTP Recovery method:
 - set computer to a static ip, 192.168.1.75
 - connect computer to the LAN 1 port of the router
 - hold the reset button while powering on the router for a few seconds
 - send firmware image using a tftp client; i.e from linux:
 $ tftp
 tftp> binary
 tftp> connect 192.168.1.1
 tftp> put lede-ramips-mt7620-rt-ac51u-squashfs-sysupgrade.bin
 tftp> quit

Signed-off-by: Ørjan Malde <foxyred333@gmail.com>