cpuidle: add MAINTAINERS entry for ARM Exynos cpuidle driver

Since there has been quite a lot of development going on for
ARM Exynos cpuidle driver recently I would like to add separate
MAINTAINERS entry for it and add myself as the primary maintainer.

The merging process would remain (almost) unchanged with patches
going (with my Ack) through Daniel's or Kukjin's tree.

Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>