ath79: add support for TP-Link RE450 v3

ath79: add support for TP-Link RE450 v3

TP-Link RE450 v3 is a dual band router/range-extender based on
Qualcomm/Atheros QCA9563 + QCA9880.

This device is nearly identical to RE450 v2 besides a modified flash
layout (hence I think force-flashing a RE450v2 image will lead to at
least loss of MAC address).

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-
- possible UART header on PCB¹

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

¹ Didn't check to connect as I didn't even manage to connect on
  RE450v2 (AFAIU it requires disconnecting some resistors, which I was
  too much of a coward to do).  But given the similarities to v2 I
  think it's the same or very similar procedure (and most likely also
  the only way to debrick).

Signed-off-by: Andreas Wiese <aw-openwrt@meterriblecrew.net>
[remove dts-v1 and compatible in DTSI]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>