imx: mx6: ddr add an entry ddr_type for mx6_ddr_sysinfo

Add ddr_type entry for mx6_ddr_sysinfo. It will be used for
differenrate DDR3 and LPDDR2.

Introduce an enum type for ddr_type.

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