irqchip: gic-v3: Implement CPU PM notifier

When a CPU enters a low power state, the contents of the GICv3/4 system
registers are lost. They need to be saved and restored if required.

For now, since most of the GICv3 register are set some initial values and
not modified at runtime, it is better to re-initialise rather than saving
and restoring them. It may need to be saved and restored in future if
required.

This patch adds a notifier to disable the redistributor(if allowed) and
Group1 interrupts when powering down the processor and to re-initialise
the system registers on wakeup.

Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/1409065415-20176-3-git-send-email-sudeep.holla@arm.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>