mx6cuboxi: Prepare for multi SoC support

Cubox-i and Hummingboard support several MX6 SoCs: mx6solo, mx6dual-lite,
mx6dual and mx6quad.

Use IOMUX_PADS() macro in order to prepare for the multi-SoC support.
Also pass 'MX6QDL' in the defconfig to indicate it.

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