ASoC: dt-bindings: fsl_rpmsg: Add binding doc for rpmsg audio device

fsl_rpmsg is a virtual audio device. Mapping to real hardware
devices are SAI, DMA controlled by Cortex M core. What we see from
Linux side is a device which provides audio service by rpmsg channel.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1615516725-4975-4-git-send-email-shengjiu.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>