dt-bindings: spi: Convert imx cspi to json-schema

Convert the i.MX CSPI binding to DT schema format using json-schema,
update compatible, remove obsolete properties "fsl,spi-num-chipselects"
and update the example based on latest DT file.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/1592281575-32708-3-git-send-email-Anson.Huang@nxp.com


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