firmware: arm_scmi: Use opps_by_lvl to store opps

Store all the discovered OPPs into the XArray opps_by_lvl even when
level_indexing mode is not used, since it comes handy to easily retrieve
OPPs by level.

Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Link: https://lore.kernel.org/r/20240212123233.1230090-11-cristian.marussi@arm.com


Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>