arm64: dts: imx8mm: Add Engicam i.Core MX8M Mini SoM

i.Core MX8M Mini is an EDIMM SoM based on NXP i.MX8M Mini
from Engicam.

General features:
- NXP i.MX8M Mini
- Up to 2GB LDDR4
- 8/16GB eMMC
- Gigabit Ethernet
- USB 2.0 Host/OTG
- PCIe Gen2 interface
- I2S
- MIPI DSI to LVDS
- rest of i.MX8M Mini features

i.Core MX8M Mini needs to mount on top of Engicam baseboards
for creating complete platform solutions.

Add support for it.

Signed-off-by: Matteo Lisi <matteo.lisi@engicam.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>