dt-bindings: hisilicon: Merge hi3620-clock into hisilicon,sysctrl binding

The hi3620-clock binding is simple and always a child of the
"hisilicon,sysctrl" node, so just add it into the hisilicon,sysctrl
binding and drop the old txt binding.

Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>