mediatek: add support for Acer Predator W6

mediatek: add support for Acer Predator W6

Hardware
--------
SOC:   MediaTek MT7986A
RAM:   1GB DDR4
FLASH: 4GB eMMC
WiFi:  2x2 2.4 GHz 802.11 b/g/n/ax MT7916 DBDC
       4x4 5 GHz   802.11 a/n/ac/ax MT7986
       2x2 6 GHz   802.11ax MT7916 DBDC
ETH:   4x LAN 1Gbit/s (MT7531)
       1x WAN 2.5Gbit/s (GPY211)
BTN:   RESET, WPS
LED:   Antenna LEDs (GPIO)
       Mood-LED (Kinetic KTD2601) - unsupported
UART:  Header nest to USB port - 3V3 115200 8N1
       [BUTTON] GND - RX - TX [USB]

Installation
------------

1. Connect to the device using serial console.

2. Interrupt the Autoboot process when promted by sending '0' twice.

3. Serve the OpenWrt initramfs image using TFTP at 192.168.1.66. Name
   the image "predator.bin" and conenct the TFTP server to the routers
   LAN port.

4. Configure U-Boot to allow loading unsigned images from MMC

   $ setenv bootcmd 'mmc read 0x40000000 0x00004400 0x0010000;
     fdt addr $(fdtcontroladdr); fdt rm /signature; bootm 0x40000000';
     saveenv

5. Transfer the image from U-Boot

   $ setenv serverip 192.168.1.66; setenv ipaddr 192.168.1.1;
     tftpboot 0x46000000 predator.bin; fdt addr $(fdtcontroladdr);
     fdt rm /signature; bootm

6. Wait for OpenWrt to boot

7. Transfer the OpenWrt sysupgrade image to the router using scp.

8. Install OpenWrt using sysupgrade.

Signed-off-by: David Bauer <mail@david-bauer.net>