dt-bindings: serial: samsung: add specific compatible for Tesla FSD

Tesla FSD is a derivative of Samsung Exynos SoC, thus just like the
others it reuses several devices from older designs.  Historically we
kept the old (block's) compatible only.  This works fine and there is no
bug here, however guidelines expressed in
Documentation/devicetree/bindings/writing-bindings.rst state that:
1. Compatibles should be specific.
2. We should add new compatibles in case of bugs or features.

Add Tesla FSD compatible specific to be used with an existing fallback.

Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231205092229.19135-4-krzysztof.kozlowski@linaro.org
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>