drm/exynos: gsc: add device tree support and remove usage of static mappings

This patch adds device tree support for exynos_drm_gsc. This patch
also fixed build issue on non-Exynos platforms, thus dependency on
!ARCH_MULTIPLATFORM can be now removed. The driver cannot be used
simultaneously with V4L2 Mem2Mem GScaller driver thought.

Signed-off-by: Seung-Woo Kim <sw0312.kim@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>