ARM: EXYNOS: Use exynos4 prefix instead of exynos4210 on exynos4-dt

Since mach-exynos4-dt.c is also going to be used for other SoCs from EXYNOS4
line, rename internal structures and functions to use exynos4_ prefix, instead
of exynos4210_.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>