mx7ulp_evk: Use imx_ddr_size()

Use imx_ddr_size() to retrieve the total DDR size instead
of passing a hardcoded memory size.

imx_ddr_size() calculates the memory size based on the actual
MMDC registers values and is useful to detect misconfigurations,
so switch to this more robust approach.

Signed-off-by: Fabio Estevam <festevam@gmail.com>

board/freescale/mx7ulp_evk/mx7ulp_evk.c

@@ -17,7 +17,7 @@ DECLARE_GLOBAL_DATA_PTR;
 
 int dram_init(void)
 {
-	gd->ram_size = PHYS_SDRAM_SIZE;
+	gd->ram_size = imx_ddr_size();
 
 	return 0;
 }