ramips: add support for ASUS RT-AX53U

ramips: add support for ASUS RT-AX53U

Specifications:
- Device: ASUS RT-AX53U
- SoC: MT7621AT
- Flash: 128MB
- RAM: 256MB
- Switch: 1 WAN, 3 LAN (10/100/1000 Mbps)
- WiFi: MT7905 2x2 2.4G + MT7975 2x2 5G
- Ports: USB 3.0
- LEDs: 1x POWER (blue, configurable)
        3x LAN (blue, configurable)
        1x WAN (blue, configurable)
        1x USB (blue, not configurable)
	1x 2.4G (blue, not configurable)
	1x 5G (blue, not configurable)

Flash by U-Boot TFTP method:
- Configure your PC with IP 192.168.1.2
- Set up TFTP server and put the factory.bin image on your PC
- Connect serial port(rate:115200) and turn on AP, then interrupt "U-Boot Boot Menu" by hitting any key
   Select "2. Upgrade firmware"
   Press enter when show "Run firmware after upgrading? (Y/n):"
   Select 0 for TFTP method
   Input U-Boot's IP address: 192.168.1.1
   Input TFTP server's IP address: 192.168.1.2
   Input IP netmask: 255.255.255.0
   Input file name: openwrt-ramips-mt7621-asus_rt-ax53u-squashfs-factory.bin
- Restart AP aftre see the log "Firmware upgrade completed!"

Signed-off-by: Chuncheng Chen <ccchen1984@gmail.com>
(replaced led label, added key-* prefix to buttons, added note about
BBT)
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>