PM / EM: Expose perf domain struct

In the current state, the perf_domain struct is fully defined only when
CONFIG_ENERGY_MODEL=y. Since we need to write code that compiles both
with or without that option in the thermal framework, make sure to
actually define the struct regardless of the config option. That allows
to avoid using stubbed accessor functions all the time in code paths
that use the EM.

Signed-off-by: Quentin Perret <quentin.perret@arm.com>