PM / devfreq: Add COMPILE_TEST for build coverage

The SoC-specific devfreq and devfreq-event drivers can be build tested
on all architectures.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>