mailbox: imx: clear pending interrupts

During MU initialization, there maybe pending GSR and RSR pending
interrupt, clear them to avoid unexpected kernel dump when requesting
mailbox channel

Reviewed-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Ye Li <ye.li@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>