firmware: arm_scmi: Move protocol registration helpers

Move protocol registration helpers and logic out of bus.c compilation
unit into driver.c.

No functional change.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20221222185049.737625-4-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>