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>