lpm_levels: Select mode based on steady state power for hotplug

Currently we choose power collapse or standalone power collapse
as default mode for hotplug. Instead select deepest mode based
on the least steady state power.

If lpm isn't probed yet, then check for mode availability for
this spm device and select this mode for cpu hotplug.

Change-Id: Ia54994ae4ed65af20318fdbe68095ac7177ad759
Signed-off-by: Archana Sathyakumar <asathyak@codeaurora.org>