firmware: arm_scmi: Extend perf protocol ops to get number of domains

Similar to other protocol ops, it's useful for an scmi module driver to get
the number of supported performance domains, hence let's make this
available by adding a new perf protocol callback. Note that, a user is
being added from subsequent changes.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Link: https://lore.kernel.org/r/20230825112633.236607-2-ulf.hansson@linaro.org
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>