arm64: dts: imx8mp: Remove the reference to audio ipg clock on imx8mp

On i.MX8MP, there is no audio ipg clock, so remove the wrong reference
to this clock in dts file.

Signed-off-by: Jacky Bai <ping.bai@nxp.com>
Reviewed-by: Abel Vesa <abel.vesa@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>