ASoC: imx-audio-rpmsg: Add rpmsg_driver for audio channel

This driver is used to accept the message from rpmsg audio
channel, and if this driver is probed, it will help to register
the platform driver, the platform driver will use this
audio channel to send and receive messages to and from Cortex-M
core.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1615516725-4975-5-git-send-email-shengjiu.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>