doc: power_mgmt: Update PM doc with latest changes

Revise the document to reflect the latest changes including
the updated concept of SOC interface instead of PMA. Simplified
and enhanced areas that were known to cause confusion. Added
descriptions of new APIs and usages.

Jira: ZEP-1386
Change-Id: I5fa74d85245924f512c22d9d977dd0c9ea62b6ce
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>