omap3: Configure RAM bank 0 if in SPL

OMAP3 relied on the memory config done by X-loader or Configuration Header. This
has to be reworked for the implementation of a SPL. This patch configures RAM
bank 0 if CONFIG_SPL_BUILD is set. Settings for Micron-RAM used by devkit8000
are added to mem.h

Signed-off-by: Simon Schwarz <simonschwarzcor@gmail.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>