firmware: arm_scmi: Split bus and driver into distinct modules

Make the SCMI bus on its own as a distinct module initialized at
subsys_initcall level when builtin.

Keep the SCMI driver core stack, together with any configured transport,
in a different module initialized as module_init level.

SCMI drivers initialization remain unchanged at module_init level.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20221222185049.737625-10-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>