mailbox: imx: support return value of init

There will be changes that init may fail, so adding return value for
init function.

Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>