dt-bindings: arm: fsl: support Engicam MicroGEA BMM board

Add devicetree bindings for Engicam MicroGEA BMM board based on the
Engicam MicroGEA SoM (System-on-Module).

The use of an enum for a single element is justified by the future
addition of other boards based on the same SoM.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Author: passgat

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -770,6 +770,13 @@ properties:
           - const: dh,imx6ull-dhcor-som
           - const: fsl,imx6ull
 
+      - description: i.MX6ULL Engicam MicroGEA SoM based boards
+        items:
+          - enum:
+              - engicam,microgea-imx6ull-bmm       # i.MX6ULL Engicam MicroGEA BMM Board
+          - const: engicam,microgea-imx6ull        # i.MX6ULL Engicam MicroGEA SoM
+          - const: fsl,imx6ull
+
       - description: i.MX6ULL PHYTEC phyBOARD-Segin
         items:
           - enum: