ARM: EXYNOS: Decouple ARCH_EXYNOS from PLAT_S5P

After removing support for ATAGS based boot on Exynos, there is not much
that can be shared between Exynos and other S5P platforms. This patch
makes Exynos a standalone Samsung platform, not using PLAT_S5P.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>