mx6sabresd: Prepare for supporting MX6DL

Currently only MX6Q/QP sabresd boards are supported in SPL.

In order to also support MX6DL we need to convert to using
IOMUX_PADS and SETUP_IOMUX_PADS macros.

Other than that move the <asm/arch/mx6-ddr.h> header inclusion to the
SPL code block in order to avoid build error.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Jagan Teki <jagan@openedev.com>