media: dt-bindings: media: Document RDA5807 FM radio bindings

Add documentation for the devicetree bindings of the RDA5807 FM radio
I2C chip from Unisoc.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>