dt-bindings: timer: exynos4210-mct: Add missing clocks

dt-bindings: timer: exynos4210-mct: Add missing clocks

The Exynos Multi Core Timer driver requires two clocks.  This was never
documented in the binding.  Add it to fix dtschema warnings like:

  arch/arm64/boot/dts/exynos/exynos5433-tm2.dt.yaml: timer@101c0000:
    'clock-names', 'clocks' do not match any of the regexes: 'pinctrl-[0-9]+'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200903191311.12293-2-krzk@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>