ASoC: dt-bindings: fsl_rpmsg: Add compatible string for i.MX8ULP

Add compatible string for i.MX8ULP platform which support audio
function through rpmsg audio channel on M core.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Link: https://lore.kernel.org/r/1630553525-25655-1-git-send-email-shengjiu.wang@nxp.com


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