arm64: dts: imx8mn-beacon: Add DMIC support

The baseboard has a connector for a pulse density microphone.
This is connected via the micfil interface and uses the DMIC
audio codec with the simple-audio-card.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>