lpm: Allow cpu to enter FPC from hypervisor

FPC using PSCI is entered from PSCI layer that is in Secure EL1.
Switching of EL layers incur additional latency, making FPC slower.
Issue wfi within hypervisor for cpu only sleep. This makes FPC much
faster than entering from PSCI layer.

Change-Id: Icf4c5f2484fdda79c991b842cb3a3185b638bfdb
Signed-off-by: Archana Sathyakumar <asathyak@codeaurora.org>