mediatek: add GL.iNET GL-MT3600BE support

mediatek: add GL.iNET GL-MT3600BE support

Hardware specification:
  SoC: MediaTek MT7987A 4x A53
  Flash: 512 MB SPI-NAND
  RAM: 512MB DDR4
  Ethernet: 2x 2.5GbE (built-in + RTL8221B)
  WiFi: MediaTek MT7990
  USB: 1x USB 3.0 port
  Button: Mode, Reset
  Power: Type-C PD 5/9/12V

Flash instructions:
  1. Power on the device with 'reset' key pressed for 5s
  2. Set static IP on your PC:
     IP 192.168.1.10/24, GW 192.168.1.1
  3. Visit http://192.168.1.1 and upload sysupgrade.bin

Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>
Link: https://github.com/openwrt/openwrt/pull/22476
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>