ramips: add support for TP-Link Deco M4R v4

ramips: add support for TP-Link Deco M4R v4

This commit adds support for the V4 hardware revision of the Deco M4R.
V4 is a complete overhaul of the hardware compared to V1 and V2,
and is much more similar to the Archer C6 V3 and C6U V1.

Specifications:

  SoC: MediaTek MT7621AT (2 cores at 880 MHz, 4 threads)
  RAM: Kingston D1216ECMDXGJD (256 MB)
  Wireless 2.4 GHz: MediaTek MT7603EN
  Wireless 5 GHz: MediaTek MT7613BEN
  Flash: 16 MB SPI NOR

Installation:

Flash the *-factory.bin image in the U-Boot recovery webserver.
You can trigger this webserver by holding the reset button until the LED
flashes yellow, or by hooking up to serial pads on the board (clearly
labeled GND, RX and TX) and pressing `x` early in boot.

Once the factory image has been flashed, you can use the regular upgrade
procedure with sysupgrade images for subsequent flashes.

Signed-off-by: Mark Ceeha <hi@shiz.me>
Tested-by: Mark Ceeha <hi@shiz.me>