ASoC: fsl: Support register and unregister rpmsg

Merge series from Chancel Liu <chancel.liu@nxp.com>:

	echo /lib/firmware/fw.elf > /sys/class/remoteproc/remoteproc0/firmware
(A)	echo start > /sys/class/remoteproc/remoteproc0/state
(B)	echo stop > /sys/class/remoteproc/remoteproc0/state

The rpmsg sound card is registered in (A) and unregistered in (B).
After "start", imx-audio-rpmsg registers devices for ASoC platform driver
and machine driver. Then sound card is registered. After "stop",
imx-audio-rpmsg unregisters devices for ASoC platform driver and machine
driver. Then sound card is unregistered.