ARM: EXYNOS: Make ARCH_EXYNOS select USE_OF

Since EXYNOS is now DT-only, it always requires USE_OF option to be
selected. This patch moves USE_OF selection from entries of DT board
files to ARCH_EXYNOS.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>