arm64: kernel: add CPU idle call

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>