nitrogen6x: Add support for i.MX6SDL CPU variants to SOC_FAMILY

The Nitrogen6x has options for i.MX6DL and i.MX6S, and
without mx6dl in the SOC_FAMILY, Dual-Lite/Solo features
of other components won't be built.

The problem triggering this patch is the firmware-imx package,
which will not install the Dual-Lite/Solo firmware for the
VPU (lib/firmware/vpu_fw_imx6d.bin).

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
(cherry picked from commit 9d34fea1d067da7ffbca07b6f0827391aa207e7f)