mx6sabresd: Add SPL support for the mx6dl variant

Add support for the mx6dlsabresd board in SPL.

Retrieved the DCD table from:
board/freescale/mx6sabresd/mx6dlsabresd.cfg
(NXP U-Boot branch imx_v2015.04_4.1.15_1.0.0_ga)

Flashed SPL and u-boot.img to an SD card and could successfully boot it
on mx6q, mx6qp and mx6dl sabresd boards.

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