arm64: Convert cpu_do_idle() to using cpuidle context helpers

arm64: Convert cpu_do_idle() to using cpuidle context helpers

Now that we have helpers that are aware of the pseudo-NMI
feature, introduce them to cpu_do_idle(). This allows for
some nice cleanup.

No functional change intended.

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/20210615111227.2454465-3-maz@kernel.org
Signed-off-by: Will Deacon <will@kernel.org>