mx6qsabreauto: Avoid hardcoded RAM size

Instead of passing the total RAM size via PHYS_SDRAM_SIZE option,
we should better use imx_ddr_size() function, which automatically
determines the RAM size.

Signed-off-by: Vanessa Maegima <vanessa.maegima@nxp.com>
Acked-by: Fabio Estevam <fabio.estevam@nxp.com>