dt-bindings: clk: add documentation for the SiFive PRCI driver

SiFive FU540 PRCI bindings

On the FU540 family of SoCs, most system-wide clock and reset integration

is via the PRCI IP block.

Required properties:

- compatible: Should be "sifive,<chip>-prci".  Only one value is

	supported: "sifive,fu540-c000-prci"

- reg: Should describe the PRCI's register target physical address region

- clocks: Should point to the hfclk device tree node and the rtcclk

          device tree node.  The RTC clock here is not a time-of-day clock,

	  but is instead a high-stability clock source for system timers

	  and cycle counters.

- #clock-cells: Should be <1>

The clock consumer should specify the desired clock via the clock ID

macros defined in include/dt-bindings/clock/sifive-fu540-prci.h.

These macros begin with PRCI_CLK_.

The hfclk and rtcclk nodes are required, and represent physical

crystals or resonators located on the PCB.  These nodes should be present

underneath /, rather than /soc.

Examples:

/* under /, in PCB-specific DT data */

hfclk: hfclk {

	#clock-cells = <0>;

	compatible = "fixed-clock";

	clock-frequency = <33333333>;

	clock-output-names = "hfclk";

};

rtcclk: rtcclk {

	#clock-cells = <0>;

	compatible = "fixed-clock";

	clock-frequency = <1000000>;

	clock-output-names = "rtcclk";

};

/* under /soc, in SoC-specific DT data */

prci: clock-controller@10000000 {

	compatible = "sifive,fu540-c000-prci";

	reg = <0x0 0x10000000 0x0 0x1000>;

	clocks = <&hfclk>, <&rtcclk>;

	#clock-cells = <1>;

};