ath79: add support for TP-Link RE450 v2

ath79: add support for TP-Link RE450 v2

TP-Link RE450 v2 is a dual band router/range-extender
based on Qualcomm/Atheros QCA9563 + QCA9880.

Specification:

- 775 MHz CPU
- 64 MB of RAM (DDR2)
- 8 MB of FLASH (SPI NOR)
- 3T3R 2.4 GHz
- 3T3R 5 GHz
- 1x 10/100/1000 Mbps Ethernet (AR8033 PHY)
- 7x LED, 4x button
- UART header on PCB (needs unmounted R64 & R69 0201 resistors/jumpers)

Flash instruction:
Apply factory image in OEM firmware web-gui.

U-Boot does not seem to have any recovery functions, so
debricking requires connection via UART.

Signed-off-by: Peter Lundkvist <peter.lundkvist@gmail.com>