dt-bindings: rtc: Convert MXC RTC to json-schema

Convert the MXC RTC binding to DT schema format using json-schema.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>