ARM: EXYNOS: add support ARCH_EXYNOS5 for EXYNOS5 SoCs

This patch adds CONFIG_ARCH_EXYNOS5 and CONFIG_SOC_EXYNOS5250
for supporting EXYNOS5250 SoC and allows supporting EXYNOS4
and EXYNOS5 together.

Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>