dt-bindings: tegra: Document Jetson Orin NX reference platform

Document the combination of the P3768 carrier board with the P3767
(Jetson Orin NX) module.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>