dt-bindings: clock: Refine i.MX8MN clock binding

Refine i.MX8MN clock binding by removing useless content and
updating the example, it makes all i.MX8M SoCs' clock binding
aligned.

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