dt-bindings: gpu: arm,mali-midgard: add specific compatibles for existing Exynos SoC

Samsung Exynos SoC reuses several devices from older designs, thus
historically we kept the old (block's) compatible only. This works fine
and there is no bug here, however guidelines expressed in
Documentation/devicetree/bindings/writing-bindings.rst state that:
1. Compatibles should be specific.
2. We should add new compatibles in case of bugs or features.

Add compatibles specific to each SoC in front of all old-SoC-like
compatibles.

Reviewed-by: Alim Akhtar <[e-mail]>
Acked-by: Rob Herring <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Krzysztof Kozlowski <[e-mail]>

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

···
           - const: arm,mali-t760
       - items:
           - enum:
+              - samsung,exynos7-mali
+          - const: samsung,exynos5433-mali
+          - const: arm,mali-t760
+      - items:
+          - enum:
               - rockchip,rk3399-mali
           - const: arm,mali-t860