quark_se: power_mgmt: Fixes a cpu context save bug

The cpu context save function was manipulating stack and
returning to C caller. This can corrupt stack if the calling
function has data saved and it pops before entering deep
sleep. Moved sleep functions into assembly to avoid this.

Jira: ZEP-1345
Change-Id: I8a6d279ec14e42424f764d9ce8cbbef32149fe84
Signed-off-by: Ramesh Thomas <ramesh.thomas@intel.com>