dt-bindings: arm: Add Polyhex DEBIX SOM A based boards

Add devicetree bindings for i.MX8MP based DEBIX SOM A and SOM A I/O
baseboard:
  - https://debix.io/hardware/debix-som-a.html
  - https://debix.io/hardware/debix-som-a-io-board.htmlSigned-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -1077,6 +1077,13 @@ properties:
           - const: polyhex,imx8mp-debix             # Polyhex i.MX8MP Debix SBCs
           - const: fsl,imx8mp
 
+      - description: Polyhex DEBIX i.MX8MP SOM A based boards
+        items:
+          - enum:
+              - polyhex,imx8mp-debix-som-a-bmb-08   # Polyhex Debix SOM A on SOM A I/O board
+          - const: polyhex,imx8mp-debix-som-a       # Polyhex Debix SOM A
+          - const: fsl,imx8mp
+
       - description: Toradex Boards with Verdin iMX8M Plus Modules
         items:
           - enum: