arm64: dts: imx8qxp: Add audio clock mux node

The audio clock mux (ACM) selects the input clock for each attached
consumer, referenced by clock-cell.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>