ARM: dts: exynos: add input clock to CMU in Exynos3250

Commit 52005dec ("ARM: dts: Add assigned clock parents to CMU node
for exynos3250") added assigned clocks under Clock Management Unit to
fix hangs when accessing ISP registers.

However the dtschema expects "clocks" property if "assigned-clocks" are
used.  Add reference to input clock to silence the dtschema warnings:

  arch/arm/boot/dts/exynos3250-artik5-eval.dt.yaml: clock-controller@10030000: 'clocks' is a dependency of 'assigned-clocks'

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20200906142146.21266-1-krzk@kernel.org