[media] Documentation: devicetree: Add document bindings for mtk-cir

This patch adds documentation for devicetree bindings for
consumer Mediatek IR controller.

Signed-off-by: Sean Wang <sean.wang@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sean Young <sean@mess.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>