firmware: arm_scmi: Add SCMI System Power Control driver

Add an SCMI System Power control driver to handle platform's requests
carried by SYSTEM_POWER_STATE_NOTIFIER notifications: such platform
requested system-wide power state transitions are handled accordingly,
gracefully or forcefully, depending on the notifications' message flags.

Graceful requests are relayed to userspace using the same Kernel API used
to handle ACPI Shutdown bus events.

Link: https://lore.kernel.org/r/20220704101933.2981635-5-cristian.marussi@arm.com
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>