configs/freescale_imx6dlsabresd: add defconfig for iMX6DL SabreSD

This is very similar to freescale_imx6qsabresd_defconfig, which supports the
i.MX6Q ("Quad", quad core CPU) version of the board, except it supports the
i.MX6DL (DualLite, dual core CPU) version.

The differences are in the U-boot configuration and the device tree file.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Cc: Gilles Talis <gilles.talis@gmail.com>
Cc: Gary Bisson <bisson.gary@gmail.com>
Cc: Vincent Stehlé <vincent.stehle@freescale.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>