ARMV7: OMAP4: Calculate SDRAM size

Calculate the SDRAM size from DMM configuration registers instead of using
hard-coded values. This gives correct values for all different boards.

It's assumed that DMM sections do not overlap memory areas.

Signed-off-by: Aneesh V <aneesh@ti.com>
Tested-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>