remoteproc: imx_dsp_rproc: Use devm_rproc_add() helper

remoteproc: imx_dsp_rproc: Use devm_rproc_add() helper

Replace manual rproc_add() and cleanup logic with devm_rproc_add(), which
ties the remoteproc lifecycle to the device's lifecycle. This simplifies
error handling and ensures proper cleanup.

No functional changes.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Reviewed-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reviewed-by: Iuliana Prodan <iuliana.prodan@nxp.com>
Tested-by: Iuliana Prodan <iuliana.prodan@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20251119-imx-dsp-2025-11-19-v4-2-adafd342d07b@nxp.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>