ARM: EXYNOS: add support for EXYNOS5250 SoC

This patch add support for EXYNOS5250 SoC has two Cortex-A15 cores.
Since actually, most codes in mach-exynos/ are used commonly for
EXYNOS4 and EXYNOS5 the EXYNOS5/EXYNOS5250 has been implemented
in mach-exynos/.

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