ARM: EXYNOS: add platform device for core DRM subsystem

Add platform device for Exynos DRM core. This device is used
for creating DRM user-space device and allocating memory for
display buffers.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>