ARM: 7176/1: cpu_pm: register GIC PM notifier only once

When multiple GICs exist on a platform (RealView PB1176/11MP),
we must make sure the PM notifier block is only registered
once, otherwise we end up corrupting the PM notifier list.

The fix is to only register the notifier when initializing
the first GIC, as the power management functions seem
to iterate over all the registered GICs.

Tested on PB11MP and PB1176.

Reported-by: Will Deacon <will.deacon@arm.com>
Tested-by: Will Deacon <will.deacon@arm.com>
Cc: Colin Cross <ccross@android.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>