dt-bindings: zte: add bindings document for zx2967 power domain controller

* ZTE zx2967 family Power Domains

zx2967 family includes support for multiple power domains which are used

to gate power to one or more peripherals on the processor.

Required Properties:

  - compatible: should be one of the following.

      * zte,zx296718-pcu - for zx296718 power domain.

  - reg: physical base address of the controller and length of memory mapped

      region.

  - #power-domain-cells: Must be 1.

Example:

	pcu_domain: pcu@117000 {

		compatible = "zte,zx296718-pcu";

		reg = <0x00117000 0x1000>;

		#power-domain-cells = <1>;

	};