dt-bindings: arm: samsung: Document missing Exynos7 boards bindings

Add missing documentation of ARMv8 Samsung Exynos7 SoC based boards
bindings.
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/arm/samsung/samsung-boards.yaml

@@ -171,5 +171,11 @@ properties:
               - samsung,tm2e                    # Samsung TM2E
           - const: samsung,exynos5433
 
+      - description: Exynos7 based boards
+        items:
+          - enum:
+              - samsung,exynos7-espresso        # Samsung Exynos7 Espresso
+          - const: samsung,exynos7
+
 required:
   - compatible