power_mgmt: Create arch/soc specific helper functions

Created set of functions that was designed in the original RFC to
contain arch and soc specific code.  This makes the interface to
the OS PM infrastructure cleaner and makes it easier to understand
the flow in the sample app. Also it makes code reuse easier across
soc/arch implementations.  These are open to change in future if
other means to achieve the same goals are in place.

Jira: ZEP-227 ZEP-225
Change-Id: Ief57871c370341c55009ad4f456b7f71f2c2a3c6
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>