PM: EM: add macro to set .active_power() callback conditionally

The Energy Model is able to use new power values coming from DT. Add a new
macro which is helpful in setting the .active_power() callback
conditionally in setup time. The dual-macro implementation handles both
kernel configurations: w/ EM and w/o EM built-in.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>