ramips: add support for TP-Link RE350

ramips: add support for TP-Link RE350

The TP-Link RE350 is a wall-wart AC1200 range extender/access point with
a single gigabit ethernet port and two non-detachable antennas, based on
the MT7621A SoC with MT7603E and MT7612E radios.

Firmware wise it is very similar to the QCA based RE450.

SoC: MediaTek MT7621A (880MHz)
Flash: 8MiB (Winbond W25Q64)
RAM:   64MiB (DDR2)
Ethernet: 1x 1Gbit
Wireless: 2T2R 2.4Ghz (MT7603E) and 5GHz (MT7612E)
LEDs: Power, 2.4G, 5G (blue), WPS (red and blue), ethernet link/act
(green)
Buttons: On/off, LED, reset, WPS

Serial header at J1, 57600 8n1:
Pin 1 TX
Pin 2 RX
Pin 3 GND
Pin 4 3.3V

Factory image can be uploaded directly through the stock UI.

Signed-off-by: Alex Maclean <monkeh@monkeh.net>