drm/exynos: add support for exynos5420 mixer

Add support for exynos5420 mixer IP in the drm mixer driver.

Signed-off-by: Rahul Sharma <rahul.sharma@samsung.com>
Acked-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>