exynos: Move common smdk5420 things to common file

A few things are common but are not in the common file. Fix this and
rename the file to fit with the other exynos*-common files.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>
3 files changed