pm: Implement function stubs when PM is not enabled

Create stubs for when the subsystem is not compiled. Some drivers need
to use constraints when are in the middle of a transaction to avoid
the system to sleep. As the same driver may be used in a target that
does not support power management, they have to conditionally (#if
CONFIG_PM) call these functions, these stubs avoid this regular problem.

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