drm/exynos: mic: Add runtime PM support

This patch adds runtime support calls to notify device core when MIC
device is really in use. Runtime PM is implemented by enabling and
disabling clocks like in other Exynos DRM subdrivers. Adding runtime
PM support is needed to let power domain with this device to be turned
off when display is not used.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>