dt-bindings: reset: Drop the hisilicon,hi6220-reset binding

The hisilicon,hi6220-sysctrl, hisilicon,hi6220-mediactrl, and
hisilicon,hi6220-aoctrl controllers are already described in:

  Documentation/devicetree/bindings/arm/hisilicon/controller/sysctrl.yaml
  Documentation/devicetree/bindings/arm/hisilicon/controller/hi6220-domain-ctrl.yaml

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220407145802.4060130-1-p.zabel@pengutronix.de