drm/exynos: add support for ARCH_MULTIPLATFORM

Exynos does not seem to have any dependency on anything from
platform headers so just needs Kconfig updated to build in
ARCH_MULTIPLATFORM builds.

Signed-off-by: Rob Clark <rob@ti.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>