remoteproc: imx_rproc: Use devm_add_action_or_reset() for mailbox cleanup

remoteproc: imx_rproc: Use devm_add_action_or_reset() for mailbox cleanup

Convert imx_rproc_free_mbox() to a devm-managed cleanup action using
devm_add_action_or_reset(). Ensure the mailbox resources are freed
automatically with the device lifecycle, simplify error handling and
removing the need for manual cleanup in probe and remove paths.

Also improve error reporting by using dev_err_probe() for consistency and
clarity.

No functional changes.

Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20250926-imx_rproc_v3-v3-3-4c0ec279cc5f@nxp.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>