ARM: EXYNOS: Migrate Exynos specific macros from plat to mach

Move Exynos specific macros to mach-exynos from plat-samsung to avoid
unnecessary dependency on plat based header files.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Reviewed-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>