dt-bindings: dma: qcom: gpi: add compatible for sm8550

The Qualcomm SM8550 uses GPI DMA for its GENI interface. Add a compatible
string for it in the documentation by using the SM6350 as fallback.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>