drm/exynos: hdmi: enable exynos 4210 and 4x12 soc support

Configuration sets for Exynos 4210 and 4x12 SoC were already defined in
Exynos HDMI and Mixed drivers, but they lacked proper linking to device
tree 'compatible' values. This patch fixes this issue adding support for
following compatible values: samsung,exynos4210-mixer,
samsung,exynos4212-mixer and samsung,exynos4210-hdmi. It also corrects
access to sclk_mixer clock, which is available only on Exynos 4210.

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