PM / cpuidle: Add driver reference counter

PM / cpuidle: Add driver reference counter

Add a reference counter for the cpuidle driver, so that it can't
be unregistered when it is in use.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
(cherry picked from commit 6e797a078824b30afbfae6cc4b1c2b21c51761ef)

Change-Id: I41efc4cd5f99cbcfd1431ca2ac47e9aac1a11127
Signed-off-by: Prashant Gaikwad <pgaikwad@nvidia.com>
Reviewed-on: http://git-master/r/145221
Reviewed-by: Automatic_Commit_Validation_User
GVS: Gerrit_Virtual_Submit
Reviewed-by: Bharat Nihalani <bnihalani@nvidia.com>