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

Add DT binding documentation for the Linux driver for the SiFive
PRCI clock & reset control IP block, as found on the SiFive
FU540 chip.

This version includes changes requested by Stephen Boyd
<[e-mail]> and Rob Herring <[e-mail]>, and
fixes some errors in the initial version.

Signed-off-by: Paul Walmsley <[e-mail]>
Signed-off-by: Paul Walmsley <[e-mail]>
Cc: Michael Turquette <[e-mail]>
Cc: Stephen Boyd <[e-mail]>
Cc: Rob Herring <[e-mail]>
Cc: Mark Rutland <[e-mail]>
Cc: Palmer Dabbelt <[e-mail]>
Cc: Megan Wachs <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Cc: [e-mail]
Reviewed-by: Rob Herring <[e-mail]>
Signed-off-by: Stephen Boyd <[e-mail]>

Documentation/devicetree/bindings/clock/sifive/fu540-prci.txt

···
+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>;
+};