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>