Add the device-tree source files for the Tegra194 Jetson Xavier NX
Developer Kit. The Xavier NX Developer Kit consists of a small form
factor system-on-module (SOM) board (part number p3668-0000) and a
carrier board (part number p3509-0000).

The Xavier NX Developer Kit SOM features a micro-SD card slot, however,
there is also a variant of the SOM available that features a 16GB eMMC.
Given that the carrier board can be used with the different SOM
variants, that have different part numbers, both the compatible string
and file name of the device-tree source file for the Developer Kit is a
concatenation of the SOM and carrier board part numbers.

Based on some initial work by Thierry Reding <treding@nvidia.com>.
Signed-off-by: NJon Hunter <jonathanh@nvidia.com>
Signed-off-by: NThierry Reding <treding@nvidia.com>