mod_scmi_system_power_req: Add missing doxygen labels

Doxygen labels are missing for the header file.
Let's add them.
That will help with grouping the module's documentation within
the module group.

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: I69dcc17467bca796fe1751810ebc7e50a8ef1539