mediatek: add support for the GL.iNet GL-MT3000

mediatek: add support for the GL.iNet GL-MT3000

The MT-3000 is a pocket-sized Wi-Fi 6 router based on MediaTek MT7981.

Specification:
 - SoC: MediaTek MT7981B
 - CPU: 2x 1.3 GHz Cortex-A53
 - Flash: 256 MiB Macronix SPI NAND
 - RAM: 512 MiB Nanya DDR4
 - WLAN: 2.4 GHz, 5 GHz (MediaTek MT7976CN)
 - Ethernet:
    - 1x 10/100/1000 Mbps built-in PHY (LAN)
    - 1x 10/100/1000/2500 Mbps MaxLinear GPY211 PHY (WAN)
 - USB 3.0 port
 - Buttons: 1 button, 1 switch
 - LEDs: 1x light-blue, 1x warm-white
 - Serial console: internal 4-pin header, 115200 8n1
 - PWM controlled fan with tacho
 - Power: 5 VDC, 3 A (USB Type-C)

Signed-off-by: Daniel Golle <daniel@makrotopia.org>