MLK-17623: imx8 scu: Explicitly make irq optional

MLK-17623: imx8 scu: Explicitly make irq optional

The MU works just fine without interrupts because sc_call_rpc will poll
waiting for a response. Make this explicit because it allows easier
emulation for virtualization.

The request_irq error is just reported but doesn't fail the probe,
however failing to set that irq as a wake source is fatal.

This was introduced recently:
commit 3b20aa779f33 ("MLK-17072-1: soc: imx: sc: ipc: enable MU
interrupt as wakeup source")

Signed-off-by: Leonard Crestez <leonard.crestez@nxp.com>
Acked-by: Peng Fan <peng.fan@nxp.com>