cgtqmx6eval: Remove uneeded PHYS_SDRAM_SIZE

cgtqmx6eval uses the imx_ddr_size() function to calculate the DDR size in
runtime, so there is no need to define PHYS_SDRAM_SIZE.

Remove the unneeded definition.

Cc: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Acked-by: Otavio Salvador <otavio@ossystems.com.br>