imx:mx6ul add dram spl configuration and header file

1. Define two structures mx6ul_iomux_ddr_regs and mx6ul_iomux_grp_regs.
2. Add a new function mx6ul_dram_iocfg to configure dram io.
3. Refactor MMDC1 macro, discard "#ifdef CONFIG_MX6SX". Since
   only mmdc0 channel exists on i.MX6SX/UL, redefine MMDC1 macro support
   runtime check, but not hardcoding #ifdef macros.
4. Introduce mx6ul-ddr.h, which includes the register address for DRAM
   IO configuration.

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