ASoC: dt-bindings: fsl_rpmsg: Add a property to assign the rpmsg channel

Add a string property to assign the rpmsg channel this sound card sits
on. This property can be omitted if there is only one sound card and it
sits on "rpmsg-audio-channel".

Signed-off-by: Chancel Liu <chancel.liu@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220930064441.2548505-2-chancel.liu@nxp.com


Signed-off-by: Mark Brown <broonie@kernel.org>