power_mgmt: Make names consistent with new RFC

Changed names of Kconfig flags, variables, functions, files and
return codes consistent with names used in the RFC. Updated
relevant comments to match the changes.

Origin: Original
Change-Id: Ie7941032d7ad7af61fc02928f74538745e7966e8
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>
Signed-off-by: Anas Nashif <anas.nashif@intel.com>