dt-bindings: fpga: xilinx-spi: convert bindings to json-schema

Convert xilinx-spi bindings to DT schema format using json-schema.

Signed-off-by: Nava kishore Manne <nava.kishore.manne@amd.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230227110213.291225-1-nava.kishore.manne@amd.com


Signed-off-by: Rob Herring <robh@kernel.org>