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.

Change-Id: I02093e3fef946979acb28d735224d01142c63c5f
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Git-commit: b8824dfe1b3fe1d4ef3d7ee78a071b8290d3b153
Git-Repo: git://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git


Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>