firmware: arm_scmi: Add helper to access protocol revision/version

Add an helper to access from a protocol handle, the SCMI version data
which is exposed on sysfs. Such helper will be needed by SCMI base
protocol initialization once it will be moved to new protocol handles
scheme.

Link: https://lore.kernel.org/r/20210316124903.35011-10-cristian.marussi@arm.com


Tested-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>