imx:mx6sx add dram io configure for mx6sx

Define two structure mx6sx_iomux_ddr_regs and mx6sx_iomux_grp_regs.
Add a new function mx6sx_dram_iocfg to configure dram io.

Since mx6sx only have one channel mmdc0, define a new empty macro MMDC1
to replace mmdc1->entry=value for mx6sx. And to other mx6 soc, MMDC1
effects as "mmdc1->entry=value".

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>