mediatek: filogic: add Asus ZenWiFi BT8

mediatek: filogic: add Asus ZenWiFi BT8

Hardware
--------
MediaTek MT7988D SoC (3x Cortex-A53 @1.8 GHz max)
1GB DDR4 RAM
256MB SPI-NAND (Winbond)
MediaTek MT7996 BE14000 Tri-Band Wi-Fi 7
3x LAN (2x 1GE MT7988 built-in, 1x 2.5GE MaxLinear GPY211C)
1x WAN (2.5GE MT7988 built-in)
LED: RGB PWM (supported as 3x PWM LED)
USB: 1x USB 3
Buttons: RESET, WPS
UART: 115200 8N1 3.3V

Installation
------------
1. Hold down RESET button and power on the device until
   LED pulses red.

2. Assign IP 192.168.1.70/24 to your computer's Ethernet port

3. Connect Ethernet to one of the 1GE LAN ports

4. Open browser and visit http://192.168.1.1

5. Upload openwrt-mediatek-filogic-asus_zenwifi-bt8-factory.bin

6. Once OpenWrt initramfs system comes up, do sysupgrade using
   openwrt-mediatek-filogic-asus_zenwifi-bt8-squashfs-sysupgrade.bin

Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Daniel Golle <daniel@makrotopia.org>