dt-bindings: arm: samsung: Do not require clkout on Exynos5260 and Exynos7

The PMU (Power Management Unit) driver is a clkout clock provider (for
clock signal monitoring) only for certain Exynos SoCs.  It was never
implemented for Exynos5260 and Exynos7.  This fixes dtschema validator
warnings like:

    system-controller@105c0000: '#clock-cells' is a required property
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/arm/samsung/pmu.yaml

@@ -85,12 +85,28 @@ properties:
 required:
   - compatible
   - reg
-  - '#clock-cells'
-  - clock-names
-  - clocks
 
 additionalProperties: false
 
+allOf:
+  - if:
+      properties:
+        compatible:
+          contains:
+            enum:
+              - samsung,exynos3250-pmu
+              - samsung,exynos4210-pmu
+              - samsung,exynos4412-pmu
+              - samsung,exynos5250-pmu
+              - samsung,exynos5410-pmu
+              - samsung,exynos5420-pmu
+              - samsung,exynos5433-pmu
+    then:
+      required:
+        - '#clock-cells'
+        - clock-names
+        - clocks
+
 examples:
   - |
     #include <dt-bindings/clock/exynos5250.h>