spi: dt-bindings: Convert spi-sifive binding to json-schema

Convert the spi-sifive binding to DT schema format.

Signed-off-by: Pragnesh Patel <pragnesh.patel@sifive.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1568804927-13565-1-git-send-email-pragnesh.patel@sifive.com


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