thermal: cpu_cooling: Make the power-related code depend on IPA

The core CPU cooling infrastructure has power-related functions
that have only one client: IPA. Since there can be no user of those
functions if IPA is not compiled in, make sure to guard them with
checks on CONFIG_THERMAL_GOV_POWER_ALLOCATOR to not waste space
unnecessarily.

Suggested-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Quentin Perret <quentin.perret@arm.com>