arm64: Add cpuidle context save/restore helpers

arm64: Add cpuidle context save/restore helpers

As we need to start doing some additional work on all idle
paths, let's introduce a set of macros that will perform
the work related to the GICv3 pseudo-NMI idle entry exit.

Stubs are introduced to 32bit ARM for compatibility.
As these helpers are currently unused, there is no functional
change.

Tested-by: Valentin Schneider <valentin.schneider@arm.com>
Reviewed-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210615111227.2454465-2-maz@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>