PM / devfreq: exynos: Enable building exynos PPMU as module

Export symbols from the PPMU driver needed to build the exynos bus
driver as a module.

Cc: MyungJoo Ham <myungjoo.ham@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Kukjin Kim <kgene.kim@samsung.com>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>