Add the clock binding doc for i.MX8MN.
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>