arm64: dts: imx8mn: Add support for micfil

The i.MX8M Nano has supports the MICFIL digital interface.
It's a 16-bit audio signal from a PDM microphone bitstream.
The driver is already in the kernel, but the node is missing.

Add the micfil node.

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