[media] Documentation: devicetree: meson-ir: "linux,rc-map-name" is supported

The driver already parses the "linux,rc-map-name" property. Add this
information to the documentation so .dts maintainers don't have to look
it up in the source-code.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.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>