firmware: arm_scmi: Add support for system suspend in power control driver

firmware: arm_scmi: Add support for system suspend in power control driver

SCMI supports system suspend notification from the platform. The suuport
for the same can be added in SCMI power control driver. However, currently
there is no way to pass suspend level to pm_suspend() call from this
driver, so use suspend-to-ram(S2R) will be used.

Couple of things to note:
1) The userspace can still configure whatever default behaviour expected
   for S2R.
2) The userspace needs to keep the wakeup source enabled, otherwise the
   system may never resume back.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20240428075105.2187837-1-peng.fan@oss.nxp.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>