imx: Place imx_ddr_size() into a separate file

Place imx_ddr_size() into a separate file.

The motivation for doing this is to be able to easily reuse
imx_ddr_size() on i.MX7ULP.

Currently imx_ddr_size() is inside arch/arm/mach-imx/cpu.c, which
is not built for i.MX7ULP.

Changing the logic to allow building cpu.c for i.MX7UP would
require adding several ifdef's, leading to a not a very elegant
solution.

To allow better reuse, just place imx_ddr_size() into a common
mmdc_size.c file.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Stefano Babic <sbabic@denx.de>