media: dt-bindings: add bindings document for zx-irdec

It adds the dt-bindings document for ZTE ZX IRDEC remote control
block.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
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>