arm64: dts: imx8-ss-audio: configure dsp node for rproc usage

The 'dsp' node is currently configured for running an audio firmware.
Since the firmware requires that some audio-related IPs be disabled in
Linux, this will clash with the configurations from the imx8 board DTs
(i.e: imx8qm-mek, imx8qxp-mek, etc...), thus making the dsp unusable
(since the firmware won't function properly).

To avoid this issue, configure the 'dsp' node for rproc. This way, users
will be able to use the dsp alongside the board-level audio configuration
as long as the firmware doesn't need the audio IPs.

Signed-off-by: Laurentiu Mihalcea <laurentiu.mihalcea@nxp.com>
Tested-by: Iuliana Prodan <iuliana.prodan@nxp.com>
Reviewed-by: Iuliana Prodan <iuliana.prodan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>