colibri_imx7: prime get_ram_size() using imx_ddr_size()

colibri_imx7: prime get_ram_size() using imx_ddr_size()

Rather than passing a hardcoded maxsize to the generic get_ram_size()
function use the i.MX 7 specific imx_ddr_size() function, which extracts
the memory size at runtime by reading the DDR controller registers.

This is a purely cosmetic change as the generic get_ram_size() function
already took care of properly automatically detecting 256MB, 512MB or 1GB
modules.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Stefan Agner <stefan.agner@toradex.com>