drm/exynos: depend on ARCH_EXYNOS for DRM_EXYNOS

Because PLAT_SAMSUNG isn't include exynos SoCs for arm64, but
ARCH_EXYNOS can do it. And it also needs to add ARCH_S3C64XX instead of
PLAT_SAMSUNG.
Signed-off-by: Joonyoung Shim <jy0922.shim@samsung.com>
Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>

drivers/gpu/drm/exynos/Kconfig

 config DRM_EXYNOS
 	tristate "DRM Support for Samsung SoC EXYNOS Series"
-	depends on OF && DRM && (PLAT_SAMSUNG || ARCH_MULTIPLATFORM)
+	depends on OF && DRM && (ARCH_S3C64XX || ARCH_EXYNOS || ARCH_MULTIPLATFORM)
 	select DRM_KMS_HELPER
 	select DRM_KMS_FB_HELPER
 	select FB_CFB_FILLRECT