imx: mx6: ddr: add LPDDR2 support

imx: mx6: ddr: add LPDDR2 support

Add LPDDR2 support:
1. Implement a function mx6_lpddr2_cfg to initialize MMDC for LPDDR2.
2. Introduce a structure mx6_lpddr2_cfg, most entrys are same to
   mx6_ddr3_cfg, but still keep it a single one for easy to choose
   parameters for LPDDR2.
3. If ddr_type is LPDDR2, use mx6_lpddr2_cfg to init MMDC.
4. Update comments.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>