samples: power_mgmt: Cleanup and update with new pm interface

Cleaned up and removed some unnecessary code to avoid
distraction from main sample implementation. Updated some
logic based on new PM interface in soc area. Updated README
to indicate it supports x86 and ARC and updated sample
output of both architectures.

Change-Id: I1c9c8348dae403b7ca6fe17ab867e3fbef06ae60
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>