Exynos: Split 5250 and 5420 memory bank configuration

Since snow has a different memory configuration than peach, split the
configuration between the 5250 and 5420. Exynos 5420 supports runtime
memory configuration detection, and can make the determination between 4
and 7 banks at runtime.

Include the bank size with the number of banks for context to make the
number of banks meaningful.

Signed-off-by: Michael Pratt <mpratt@chromium.org>
Signed-off-by: Akshay Saraswat <akshay.s@samsung.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>