firmware: arm_scmi: Add versions and identifier attributes using dev_groups

Platform drivers now have the option to have the platform core create
and remove any needed sysfs attribute files. Using the same, let's add
the scmi firmware and protocol version attributes as well as vendor and
sub-vendor identifiers to sysfs.

It helps to identify the firmware details from the sysfs entries similar
to ARM SCPI implementation.

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