In GICv3 mode, the non secure group1 interrupts are signalled via the
FIQ line in EL3. To support waking up from CPU_SUSPEND to standby on
these systems, EL3 should route FIQ to EL3 temporarily before wfi and
restore the original setting after resume. This patch makes this change
for the CSS platforms in the `css_cpu_standby` psci pm ops hook.

Change-Id: Ibf3295d16e2f08da490847c1457bc839e1bac144