bx50v3: Use imx_ddr_size() for calculating the DDR size

imx_ddr_size() can be used to calculate the DDR size in runtime.

By using this function we no longer need to define PHYS_SDRAM_SIZE.

Cc: Martin Donnelly <martin.donnelly@ge.com>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>