dt-bindings: arm: fsl: Add back missing i.MX7ULP binding

In the conversion to DT schema, the addition of the i.MX7ULP binding got
dropped. Add it to the binding schema.

Fixes: a1a38e1f ("dt-bindings: arm: Convert FSL board/soc bindings to json-schema")
Cc: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: NRob Herring <robh@kernel.org>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>

Documentation/devicetree/bindings/arm/fsl.yaml

@@ -176,6 +176,12 @@ properties:
           - const: compulab,cl-som-imx7
           - const: fsl,imx7d
 
+      - description: i.MX7ULP based Boards
+        items:
+          - enum:
+              - fsl,imx7ulp-evk           # i.MX7ULP Evaluation Kit
+          - const: fsl,imx7ulp
+
       - description: i.MX8MM based Boards
         items:
           - enum: