dt-bindings: arm: fsl: fix DEBIX binding

The current imx8mp-debix-model-a.dts uses all three compatibles. Fix the
corresponding bindings by adding an own entry for it. Adapt the comment
for the "polyhex,imx8mp-debix" binding to make it clear that this should
only be used for DEBIX Model A/B i.MX8MP SBCs.
Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -1037,8 +1037,6 @@ properties:
               - gateworks,imx8mp-gw73xx-2x # i.MX8MP Gateworks Board
               - gateworks,imx8mp-gw74xx   # i.MX8MP Gateworks Board
               - gateworks,imx8mp-gw7905-2x # i.MX8MP Gateworks Board
-              - polyhex,imx8mp-debix      # Polyhex Debix boards
-              - polyhex,imx8mp-debix-model-a # Polyhex Debix Model A Board
               - toradex,verdin-imx8mp     # Verdin iMX8M Plus Modules
               - toradex,verdin-imx8mp-nonwifi  # Verdin iMX8M Plus Modules without Wi-Fi / BT
               - toradex,verdin-imx8mp-wifi  # Verdin iMX8M Plus Wi-Fi / BT Modules
@@ -1072,6 +1070,13 @@ properties:
           - const: phytec,imx8mp-phycore-som         # phyCORE-i.MX8MP SoM
           - const: fsl,imx8mp
 
+      - description: Polyhex DEBIX i.MX8MP based SBCs
+        items:
+          - enum:
+              - polyhex,imx8mp-debix-model-a        # Polyhex Debix Model A Board
+          - const: polyhex,imx8mp-debix             # Polyhex i.MX8MP Debix SBCs
+          - const: fsl,imx8mp
+
       - description: Toradex Boards with Verdin iMX8M Plus Modules
         items:
           - enum: