ARM: EXYNOS4: convert boot_params to atag_offset

Based on "ARM: introduce atag_offset to replace boot_params"
by Nicolas Pitre (2bb9839e).

Since boot_params variable is deleted from machine_desc, the variable
is modified in the newer board files.

Cc: Nicolas Pitre <nicolas.pitre@linaro.org>
Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
[kgene.kim@samsung.com: added fixing for smdkv310]
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>