dt-bindings: qcom: qcom,geni-se: refer to dtschema for SPI

After adding DT schema for the SPI controller, the Qualcomm GENI Serial
Engine QUP Wrapper Controller bindings can reference it directly for
full schema validation.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Kuldeep Singh <singh.kuldeep87k@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220404064017.68634-2-krzysztof.kozlowski@linaro.org