firmware: arm_scmi: Refactor protocol device creation

Move protocol device request helpers from driver.c compilation unit to
bus.c, so reducing the cross interactions between driver.c and bus.c.

Get rid of old protocol device creation process as a whole from driver.c
and remove also stale SCMI system power unicity checks.

While at that make such helpers call into scmi_requested_devices_nh
notification chain.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20221222185049.737625-6-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>