arm64: dts: imx8mn: Init rates and parents configs for clocks

Add the initial configuration for clocks that need default parent and rate
setting.

NoC sources from SYS PLL3, running at 600MHz. Audio AHB/IPG clks needs
to run at 400MHz for better performance.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>