mailbox: imx: keep MU irq working during suspend/resume

During noirq suspend phase, mailbox MU irq will be masked
but many drivers still need to communicate with system
controller firmware via mailbox, if MU irq is masked, it
will cause RPC timeout as below:

[   23.372103] imx-scu scu: RPC send msg timeout

Setting MU irq to be wakeup source is NOT working as GIC
driver does NOT have .irq_set_wake implemented, so to
support suspend/resume, just make imx mailbox driver NOT
suspend, since MU is always a wakeup source on i.MX platforms
with system controller inside, and its power/clock is
maintained by system controller, mailbox driver no need
to manage them.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>