Documentation: arm64: add SCU dts binding documentation to linux kernel

APM X-GENE SoC series SCU Registers

This system clock unit contain various register that control block resets,

clock enable/disables, clock divisors and other deepsleep registers.

Properties:

 - compatible : should contain two values. First value must be:

		   - "apm,xgene-scu"

		second value must be always "syscon".

 - reg : offset and length of the register set.

Example :

	scu: system-clk-controller@17000000 {

		compatible = "apm,xgene-scu","syscon";

		reg = <0x0 0x17000000 0x0 0x400>;

	};