firmware: arm_scmi: Move handle get/set helpers

Move handle get/set helpers definitions into driver.c and invoke them
through the bus notifier helper.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20221222185049.737625-7-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>