spl: sandbox: initialise the ram banksize in spl

Initialise the ram bank information for sandbox in SPL. The ram bank
information gets initialised as part of the SPL initialisation
sequence in board_init_r(), which is then used for adding available
memory to the LMB memory map.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>