arm64: dts: imx8mm: Add support for emtrion emCON-MX8M Mini

This patch adds support for the emtrion GmbH emCON-MX8M Mini modules.
They are available with NXP i.MX 8M Mini equipped with 2 or 4 GB Memory.

The devicetree imx8mm-emcon.dtsi is the common part providing all
module components and the basic support for the SoC. The support for the
avari baseboard in the developer-kit configuration is provided by the
emcon-avari dts files.

Signed-off-by: Reinhold Mueller <reinhold.mueller@emtrion.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>