mx6cuboxi: Introduce multi-SoC support

Cubox-i and Hummingboard support several MX6 SoCs: mx6solo, mx6dual-lite,
mx6dual and mx6quad. Add support for the different SoC/memory sizes
combinations.

DDR initialization values were extracted from Solid-run internal U-boot.

Tested on a CuBox-i4Pro, HummingBoard-i2eX and HummingBoard-i1.

Signed-off-by: Rabeeh Khoury <rabeeh@solid-run.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Reviewed-by: Tom Rini <trini@konsulko.com>