firmware: arm_scmi: Skip protocol initialisation for additional devices

The scmi bus now supports adding multiple devices per protocol,
and since scmi_protocol_init is called for each scmi device created,
we must avoid allocating protocol private data and initialising the
protocol itself if it is already initialised.

In order to achieve the same, we can simple replace the idr pointer
from protocol initialisation function to a dummy function.

Suggested-by: Cristian Marussi <cristian.marussi@arm.com>
Reviewed-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>