dt-bindings: input: Convert imx keypad to json-schema

Convert the i.MX KEYPAD binding to DT schema format using json-schema

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Link: https://lore.kernel.org/r/1592285467-18371-2-git-send-email-Anson.Huang@nxp.com


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