cpuidle-haltpoll: select haltpoll governor

The haltpoll cpuidle driver should select the haltpoll governor, so
as to ensure that they work together.

Signed-off-by: Li RongQing <lirongqing@baidu.com>
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>