dt-bindings: Add Rockchip rk817 audio CODEC support

Create dt-binding documentation to document rk817 codec.
New property name of rockchip,mic-in-differential added to control if
the microphone is in differential mode or not.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Tested-by: Maciej Matuszczyk <maccraft123mc@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>