Since IRQF_NO_SUSPEND used for imx mailbox driver, that means this irq
can't be used for wakeup source so that can't wakeup from freeze mode.
Add pm_system_wakeup() to wakeup from freeze mode.

Fixes: b7b2796b("mailbox: imx: ONLY IPC MU needs IRQF_NO_SUSPEND flag")
Reviewed-by: NJacky Bai <ping.bai@nxp.com>
Reviewed-by: NPeng Fan <peng.fan@nxp.com>
Signed-off-by: NRobin Gong <yibin.gong@nxp.com>
Signed-off-by: NPeng Fan <peng.fan@nxp.com>
Signed-off-by: NJassi Brar <jaswinder.singh@linaro.org>