pm: Allow override constraints implementation

Implement constraint API as weak symbols so applications or platform
can override them. Constraints are a high-level abstraction to inhibit
the power subsystem of using certain power states. Some platforms can
have their own way to set/release constraints in their drivers that
are not part of Zephyr's code base.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>