[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 <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Signed-off-by: Sean Young <[e-mail]>
Signed-off-by: Mauro Carvalho Chehab <[e-mail]>

Documentation/devicetree/bindings/media/meson-ir.txt

···
  - reg		: physical base address and length of the device registers
  - interrupts	: a single specifier for the interrupt from the device
 
+Optional properties:
+ - linux,rc-map-name:	see rc.txt file in the same directory.
+
 Example:
 
 	ir-receiver@c8100480 {