ARM: EXYNOS: Fix Kconfig dependencies for device tree enabled machine files

Add config dependency for Exynos4 and Exynos5 device tree enabled machine
files on config options ARCH_EXYNOS4 and ARCH_EXYNOS5 respectively.
Enabling machine support without proper ARCH support enabled is incorrect.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>