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

Documentation/devicetree/bindings/reset/hisilicon,hi6220-reset.txt

-Hisilicon System Reset Controller
-======================================
-
-Please also refer to reset.txt in this directory for common reset
-controller binding usage.
-
-The reset controller registers are part of the system-ctl block on
-hi6220 SoC.
-
-Required properties:
-- compatible: should be one of the following:
-  - "hisilicon,hi6220-sysctrl", "syscon" : For peripheral reset controller.
-  - "hisilicon,hi6220-mediactrl", "syscon" : For media reset controller.
-  - "hisilicon,hi6220-aoctrl", "syscon" : For ao reset controller.
-- reg: should be register base and length as documented in the
-  datasheet
-- #reset-cells: 1, see below
-
-Example:
-sys_ctrl: sys_ctrl@f7030000 {
-	compatible = "hisilicon,hi6220-sysctrl", "syscon";
-	reg = <0x0 0xf7030000 0x0 0x2000>;
-	#clock-cells = <1>;
-	#reset-cells = <1>;
-};
-
-Specifying reset lines connected to IP modules
-==============================================
-example:
-
-        uart1: serial@..... {
-                ...
-                resets = <&sys_ctrl PERIPH_RSTEN3_UART1>;
-                ...
-        };
-
-The index could be found in <dt-bindings/reset/hisi,hi6220-resets.h>.