ARM: SAMSUNG: make local s5p-dev-mfc in mach-exynos

This patch moves s5p-dev-mfc from plat-samsung into mach-exynos
because it is used for only exynos no other platforms.

Cc: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>