[media] Documentation: devicetree: move shared property used by rc into a common place

Most IR drivers uses the same label to identify the
scancode/key table they used by multiple bindings and lack
explanation well. So move the shared property into a common
place and give better explanation.

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>