dt-bindings: tegra: Document compatible for IGX

Document the compatible strings used for Nvidia IGX Orin Development
kit which uses P3701 SKU8 and P3740 carrier board.

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