MLK-25649-1 firmware: imx: imx-scu-irq: Add support for identifying SCU wakeup source from sysfs

Record SCU wakeup interrupt in /sys/power/pm_wakeup_irq
The user can further identify the exact wakeup source by using the
following interface:
cat /sys/firmware/scu_wakeup_source/wakeup_src

The above will print the wake groups and the irqs that could have
contributed to waking up the kernel. For example if ON/OFF button was the
wakeup source:
cat /sys/firmware/scu_wakeup_source/wakeup_src
Wakeup source group = 3, irq = 0x1

The user can refer to the SCFW API documentation to identify all the
wake groups and irqs.

Signed-off-by: Ranjani Vaidyanathan <ranjani.vaidyanathan@nxp.com>
(cherry picked from commit d49daabf)