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

This add documentation for the SCU system clock unit device tree binding
to the kernel.
Signed-off-by: NFeng Kan <fkan@apm.com>

Documentation/devicetree/bindings/arm/apm/scu.txt

+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>;
+	};