dt-bindings: gpu: mali-midgard: add samsung exynos 5420 compatible

Add "samsung,exynos5420-mali" binding

Signed-off-by: Marian Mihailescu <[e-mail]>
Reviewed-by: Krzysztof Kozlowski <[e-mail]>
Signed-off-by: Rob Herring <[e-mail]>

Documentation/devicetree/bindings/gpu/arm,mali-midgard.yaml

···
           - const: arm,mali-t604
       - items:
           - enum:
+             - samsung,exynos5420-mali
+          - const: arm,mali-t628
+      - items:
+          - enum:
              - allwinner,sun50i-h6-mali
           - const: arm,mali-t720
       - items:
···
           - const: arm,mali-t860
 
           # "arm,mali-t624"
-          # "arm,mali-t628"
           # "arm,mali-t830"
           # "arm,mali-t880"