arm: psci: Support for OS initiated scheme

Existing PSCI implementation supports platform coordinated means of low
power modes where cluster low power modes are aggregated at the platform
level. Adding support for OS initiated scheme, where OS is responsible
for selecting cluster low power modes based on last man determination. With
OS initiated scheme, the OS can make better cluster decisions based on
wakeup times of CPUs within a cluster.

To this effect, in OS initiated schemes, the composite state ID is computed
by the idle driver before calling into the cpu_suspend API. The PSCI driver
is modified to use the composite ID to distinguish between retention and
non-retention states.

Change-Id: I7a5e6f047c5e72d89ff7d4e5f8201bdcbfefa54d
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>