IMX8MN_CLK_I2C4 and IMX8MN_CLK_UART1's index definitions are incorrect,
fix them.

Fixes: 1e80936a ("dt-bindings: imx: Add clock binding doc for i.MX8MN")
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>