drm: exynos: hdmi: add support for exynos5 mixer

This patch adds support for exynos5 mixer with device tree enabled.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Signed-off-by: Fahad Kunnathadi <fahad.k@samsung.com>
Signed-off-by: Kyungmin.park <kyungmin.park@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>