ARM: SAMSUNG: Introduce generic Exynos4 and 5 helpers

Add helpers to check for Exynos4 and 5 family of SoCs.
This will eliminate comparing long list of SoCs and make
code simpler.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>