Config: Exynos5420: Refactor SDRAM Bank and Size

Config: Exynos5420: Refactor SDRAM Bank and Size

Since, not every board may have all memory channels configured
and all available banks of DMC used, we wish to refactor configs
for Memory Bank size and numbers as per board memory config.
For Example, Peach-Pit has 2GB memory and will be using only 4 banks
but Peach-Pi has 3.5GB memory and will be using all 7 available
SDRAM banks.

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>