ARM: dts: add support for Gatwick board based on WB50N

Add support for the LoRa gateway from Laird, the RG1xx.
This board houses the WB50NBT CPU module along with a Semtech SX1301 based
concentrator card.
https://www.lairdtech.com/products/rg1xx-lora-gateway



Signed-off-by: Ben Whitten <ben.whitten@lairdtech.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>