dt-bindings: arm: Document RDA8810PL and reference boards

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>