imx: mx6sabresd discard PHYS_SDRAM_SIZE

This macro is not needed, since gd->ram_size is assigned value using
function imx_ddr_size().

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>