8xxx/ddr: add support to only compute the ddr sdram size

This patch adds fsl_ddr_sdram_size to only calculate the ddr sdram size, in
case that the DDR SDRAM is initialized in the 2nd stage uboot and should not
be intialized again in the final stage uboot.

Signed-off-by: Haiying Wang <Haiying.Wang@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>