firmware: arm_scmi: Stash version in protocol init functions

In order to avoid querying the individual protocol versions multiple
time with more that one device created for each protocol, we can simple
store the copy in the protocol specific private data and use them whenever
required.

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