uboot-mediatek: add build for Ubiquiti Networks UniFi 6 LR

uboot-mediatek: add build for Ubiquiti Networks UniFi 6 LR

Add U-Boot build for the Ubiquiti Networks UniFi 6 LR access point.
This allows updating the bootchain to modern ARM Trusted Firmware 2.4
and U-Boot 2021.04 while keeping as much of the existing flash layout
as possible (u-boot-env, factory and eeprom partitions are retained),
gaining robust recovery vs. production dual-boot mechanism.
Add info for the Winbond W25Q512JV SPI NOR flash used in the device
and wait for GPIOs to settle after reset before checking for reset
button.
I2C connected LED controller is not supported yet.
Writing large amounts of data to SPI flash currently doesn't work due
to watchdog timeout causing reset before data write completes.

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