arm64: dts: imx8mm: Add support for micfil

The i.MX8M Mini 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.

This patch adds the micfil node.

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