arm: Call idle notifiers in CPU idle

When CPU idle is enabled, the idle cal should also notifiy the
idle_notifier_call_chain of the change in status. Otherwise
some processes will think the CPU is always active.

Change-Id: I2fdfcae070d8f71b477adcb511a225f4eb8658f8
Signed-off-by: Tengfei Fan <tengfeif@codeaurora.org>