PM: EM: Consify two parameters of em_dev_register_perf_domain()

Notice that em_dev_register_perf_domain() and the functions called by it
do not update objects pointed to by its cb and cpus parameters, so the
const modifier can be added to them.

This allows the return value of cpumask_of() or a pointer to a
struct em_data_callback declared as const to be passed to
em_dev_register_perf_domain() directly without explicit type
casting which is rather handy.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://patch.msgid.link/4648962.LvFx2qVVIh@rjwysocki.net