nitrogen6x: include .dtb files for compatible variants

The Boundary Devices Nitrogen6X comes in a number of variants
with different processors, and the kernel and userspace is
compatible with the SABRE Lite (BD-SL-i.MX6), Nitrogen6_Lite,
and Nitrogen6_Max boards.

This patch adds support for these boards through the
corresponding .dtb files.

Signed-off-by: Eric Nelson <eric.nelson@boundarydevices.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>