CSS: Allow system suspend only via PSCI SYSTEM_SUSPEND API

CSS: Allow system suspend only via PSCI SYSTEM_SUSPEND API

The CSS power management layer previously allowed to suspend system
power domain level via both PSCI CPU_SUSPEND and PSCI SYSTEM_SUSPEND
APIs. System suspend via PSCI CPU_SUSPEND was always problematic to
support because of issues with targeting wakeup interrupts to
suspended cores before the per-cpu GIC initialization is done. This
is not the case for PSCI SYSTEM_SUSPEND API because all the other
cores are expected to be offlined prior to issuing system suspend and
PSCI CPU_ON explicit calls will be made to power them on. Hence the Juno
platform used to downgrade the PSCI CPU_SUSPEND request for system
power domain level to cluster level by overriding the default
`plat_psci_pm_ops` exported by CSS layer.

Given the direction the new CSS platforms are evolving, it is best to
limit the system suspend only via PSCI SYSTEM_SUSPEND API for all
CSS platforms. This patch makes changes to allow system suspend
only via PSCI SYSTEM_SUSPEND API. The override of `plat_psci_ops`
for Juno is removed.

Change-Id: Idb30eaad04890dd46074e9e888caeedc50a4b533
Signed-off-by: Soby Mathew <soby.mathew@arm.com>