firmware: arm_scmi: Add SCMI PM driver remove routine

firmware: arm_scmi: Add SCMI PM driver remove routine

Currently, when removing the SCMI PM driver not all the resources
registered with genpd subsystem are properly de-registered.

As a side effect of this after a driver unload/load cycle you get a
splat with a few warnings like this:

 | debugfs: Directory 'BIG_CPU0' with parent 'pm_genpd' already present!
 | debugfs: Directory 'BIG_CPU1' with parent 'pm_genpd' already present!
 | debugfs: Directory 'LITTLE_CPU0' with parent 'pm_genpd' already present!
 | debugfs: Directory 'LITTLE_CPU1' with parent 'pm_genpd' already present!
 | debugfs: Directory 'LITTLE_CPU2' with parent 'pm_genpd' already present!
 | debugfs: Directory 'LITTLE_CPU3' with parent 'pm_genpd' already present!
 | debugfs: Directory 'BIG_SSTOP' with parent 'pm_genpd' already present!
 | debugfs: Directory 'LITTLE_SSTOP' with parent 'pm_genpd' already present!
 | debugfs: Directory 'DBGSYS' with parent 'pm_genpd' already present!
 | debugfs: Directory 'GPUTOP' with parent 'pm_genpd' already present!

Add a proper scmi_pm_domain_remove callback to the driver in order to
take care of all the needed cleanups not handled by devres framework.

Link: https://lore.kernel.org/r/20220817172731.1185305-7-cristian.marussi@arm.com
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>