ANDROID: idle_notifier: Add generic idle notifiers

AOSP Change-ID: Idf29cda15be151f494ff245933c12462643388d5
moved x86_64 idle notifiers as generic so that they can
be used in interactive governor.

Upstream change 8e7a7ee9 ("x86/idle: Remove idle_notifier")
removed x86_64 idle notifiers altogether. This patch add
generic idle notifiers again.

Fixes: android-4.9 commit bfd2a547fc17 ("ANDROID: ARM: Call idle notifiers")
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>