firmware: arm_scmi: Fix the clock protocol supported version

Rollback currently supported SCMI clock protocol version to v2.0 since
some of the mandatory v3.0 features are indeed still not supported yet.

Fixes: b5efc28a

 ("firmware: arm_scmi: Add protocol versioning checks")
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20240109181716.2338636-1-cristian.marussi@arm.com
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>