dt-bindings: clock: Convert i.MX21 clock to json-schema

Convert the i.MX21 clock binding to DT schema format using json-schema,
can NOT find any CCM interrupt info from reference manual and DT file,
so interrupts property is removed from original binding doc.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>