mediatek: add support for Adtran SmartRG Bonanza Peak series

mediatek: add support for Adtran SmartRG Bonanza Peak series

The Bonanza Peak series are a couple of MT7986-powered 2.5 GBit/s
Wi-Fi 6 residential gateway, access point and mesh router products.

All of them come with an eMMC to boot from, are powered via USB-C and
got a USB 3.0 type-A port. All of them got a Dialog (Renesas) DA14531
Bluetooth module connected via UART. If the device was previously
running stock firmware, the BT chip's internal flash has been loaded
with firmware and it can be attached using hciattach when using
OpenWrt.

SOC: MediaTek MT7986A
RAM: 2 GiB DDR4
eMMC: 8 GiB
Bluetooth: BLE5 (DA14531)
Serial: 3.3V level, 115200 8n1 on 4-pin connector

 * SDG-8612 - Dual-band RJ-45 gateway
   2x 2.5G MaxLinear PHY for WAN port
   3x 1GE LAN ports via MT7531 switch

 * SDG-8614 - Dual-band SFP gateway
   1x SFP cage with up to 2.5G speed
   1x 2.5G MaxLinear PHY for LAN port
   3x 1GE LAN ports via MT7531 switch

 * SDG-8622 - Tri-band mesh router
   2x 2.5G MaxLinear PHY
   The MT7986 2G and 5G are used as 2G and 5G high band.
   There’s a MT7915 PCIe card for 5G low band.

 * SDG-8632 - Tri-band mesh router with 6 GHz
   2x 2.5G MaxLinear PHY
   The MT7986 serves the 2G and 6G bands.
   There’s a MT7915 PCIe card for 5G.

Installation via U-Boot serial console:
0. setup TFTP server with IP 192.168.1.10/24, place initramfs image
renamed to openwrt.XXX where XXX is the internal product number:
 SDG-8612: XXX = 412
 SDG-8614: XXX = 414
 SDG-8622: XXX = 422
 SDG-8632: XXX = 432

1. connect to the serial console and power on the device.
   Interrupt the bootloader by pressing 'st'
2. setenv boot_mode openwrt ; saveenv
3. run boot1
   Load firmware via TFTP and write to flash
4. run boot2
   Now OpenWrt initramfs should boot
5. upload sysupgrade.bin via scp to /tmp
6. sysupgrade

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