ASoC: imx-rpmsg: Add machine driver for audio base on rpmsg

The platform device is not registered by device tree or
cpu dai driver, it is registered by the rpmsg channel,
So add a dedicated machine driver to handle this case.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1615516725-4975-7-git-send-email-shengjiu.wang@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>