imx: mx6: ddr init MMDC according to ddr_type

To i.MX6, DDR3 and LPDDR2 is supported, so rename function mx6_dram_cfg
to mx6_ddr3_cfg and the original mx6_dram_cfg function only is a wrapper.
The new reimplemented function mx6_dram_cfg only invokes mx6_ddr3_cfg
when ddr_type is for DDR3. Later we can use ddr_type to initialize
MMDC for LPDDR2.

Initialize ddr_type for different boards which enable SPL.

Signed-off-by: Peng Fan <Peng.Fan@freescale.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Tim Harvey <tharvey@gateworks.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Stefan Roese <sr@denx.de>