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>

target/linux/ramips/base-files/etc/board.d/01_leds

@@ -314,6 +314,10 @@ rp-n53)
 	ucidef_set_led_netdev "eth" "Network" "$board:white:back" "eth0"
 	set_wifi_led "$board:blue:wifi"
 	;;
+rt-ac51u)
+	set_wifi_led "$board:blue:wifi"
+	set_usb_led "$board:blue:usb" "1-1"
+	;;
 rt-n14u)
 	ucidef_set_led_default "power" "power" "$board:blue:power" "1"
 	ucidef_set_led_netdev "lan" "lan" "$board:blue:lan" eth0.1

target/linux/ramips/base-files/etc/board.d/02_network

@@ -135,6 +135,10 @@ ramips_setup_interfaces()
 		;;
 	ar670w|\
 	ar725w|\
+	rt-ac51u)
+		ucidef_add_switch "switch0" \
+			"0:wan" "1:lan" "2:lan" "3:lan" "4:lan" "6t@eth0"
+		;;
 	rt-n15|\
 	wl-351)
 		ucidef_add_switch "switch0" \