dt-bindings: arm: Document RDA8810PL and reference boards

Add bindings for RDA Micro RDA8810PL SoC and below reference boards:

1. Orange Pi 2G-IoT - http://www.orangepi.org/OrangePi2GIOT/
2. Orange Pi i96 - https://www.96boards.org/product/orangepi-i96/

Cc: zhao_steven@263.net
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Olof Johansson <olof@lixom.net>

Documentation/devicetree/bindings/arm/rda.txt

+RDA Micro platforms device tree bindings
+----------------------------------------
+
+RDA8810PL SoC
+=============
+
+Required root node properties:
+
+ - compatible :  must contain "rda,8810pl"
+
+
+Boards:
+
+Root node property compatible must contain, depending on board:
+
+ - Orange Pi 2G-IoT: "xunlong,orangepi-2g-iot"
+ - Orange Pi i96: "xunlong,orangepi-i96"