mediatek: add support for Ubiquiti UniFi 6 LR

mediatek: add support for Ubiquiti UniFi 6 LR

Hardware
--------

MediaTek MT7622
512MB DDR3 RAM
64M SPI-NOR Flash (Winbond W25Q512JV)
MediaTek MT7622 802.11bgn 4T4R WMAC
MediaTek MT7915 802.11ax 4T4R
Marvell AQR1112 100/1000/2500 NBase-T PHY
Holtek HT32F52241 LED controller
Reset Switch

UART
----

CPU UART0 at the pinout next to the Holtek MCU.

Pinout (first pin next to SoC / MCU)

0 3V3
1 RX
2 TX
3 GND

Settings are 115200 8N1.

Opening the case
----------------

Opening the case is not a nice task, as itis glued together. Insert a
flat knife between the front and back casing below the ethernet port.
Open up a gap this way and insert a flat scredriver, remove the knife.

Work your way around the casing by applying force to seperate the front
and back casing. This losens the glue and opens the plastic clips. Be
gentle, as these clips are very cheap and break quickly.

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

1. Connect to the booted device at 192.168.1.20 using username/password
   "ubnt".

2. Transfer the OpenWrt sysupgrade image to the device using SCP.

3. Check the mtd partition number for bs / kernel0 / kernel1

   $ cat /proc/mtd

4. Set the bootselect flag to boot from kernel0

   $ dd if=/dev/zero bs=1 count=1 of=/dev/mtdblock6

5. Write the OpenWrt sysupgrade image to both kernel0 as well as kernel1

   $ dd if=openwrt.bin of=/dev/mtdblock8
   $ dd if=openwrt.bin of=/dev/mtdblock9

6. Reboot the device. It should boot into OpenWrt.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 634c13c186646aff2badb51a43b248825d0fe5a0)