mx6sxsabresd: 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>
diff --git a/board/freescale/mx6sxsabresd/mx6sxsabresd.c b/board/freescale/mx6sxsabresd/mx6sxsabresd.c
index 25e009e..8d95c51 100644
--- a/board/freescale/mx6sxsabresd/mx6sxsabresd.c
+++ b/board/freescale/mx6sxsabresd/mx6sxsabresd.c
@@ -64,7 +64,7 @@
 
 int dram_init(void)
 {
-	gd->ram_size = PHYS_SDRAM_SIZE;
+	gd->ram_size = imx_ddr_size();
 
 	return 0;
 }