When CPU idle is enabled, the architectural idle call should go through
the idle subsystem to allow CPUs to enter idle states defined
by the platform CPU idle back-end operations.

This patch, mirroring other archs behaviour, adds the CPU idle call to the
architectural arch_cpu_idle implementation for arm64.
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>