dt-bindings: serial: Convert MXS auart to json-schema

Convert the MXS auart binding to DT schema format using json-schema.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Link: https://lore.kernel.org/r/1597721685-9280-4-git-send-email-Anson.Huang@nxp.com


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