spi: xilinx: convert to yaml

Convert SPI for Xilinx bindings documentation to YAML schemas.

Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210605002931.858031-1-iwamatsu@nigauri.org


Signed-off-by: Mark Brown <broonie@kernel.org>