hda: add phytium hda driver document

* Phytium HDA controller

The HDA bus (High Definition Audio sound bus) is a serial link for digital

audio data transfer between devices in the system.

Required properties:

- compatible: should be "phytium,hda"

- reg: physical base address and length of HDA controller.

- interrupts: interrupt for the hda controller.

- clocks: phandle to clock provider with the clock number in the second cell.

- clock-name: the name of the device clock.

Example for D2000 HDA controller:

hda: hda@28206000 {

	compatible = "phytium,hda";

	reg = <0 0x28206000 0x0 0x1000>;

	interrupts = <GIC_SPI 23 IRQ_TYPE_LEVEL_HIGH>;

	clocks = <&sysclk_48mhz>;

	clock-names = "phytium_hda_clk";

};