dt-bindings: fu740: prci: add YAML documentation for the FU740 PRCI

Add YAML DT binding documentation for the SiFive FU740 PRCI. The
link of unmatched board as follow, the U740-C000 manual would be present
in the same page later.

    https://www.sifive.com/boards/hifive-unmatched

Passes dt_binding_check.

Signed-off-by: Zong Li <zong.li@sifive.com>
Link: https://lore.kernel.org/r/20201126030043.67390-1-zong.li@sifive.com
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>

Documentation/devicetree/bindings/clock/sifive/fu740-prci.yaml

@@ -0,0 +1,60 @@
+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+# Copyright (C) 2020 SiFive, Inc.
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/clock/sifive/fu740-prci.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: SiFive FU740 Power Reset Clock Interrupt Controller (PRCI)
+
+maintainers:
+  - Zong Li <zong.li@sifive.com>
+  - Paul Walmsley  <paul.walmsley@sifive.com>
+
+description:
+  On the FU740 family of SoCs, most system-wide clock and reset integration
+  is via the PRCI IP block.
+  The clock consumer should specify the desired clock via the clock ID
+  macros defined in include/dt-bindings/clock/sifive-fu740-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.
+
+properties:
+  compatible:
+    const: sifive,fu740-c000-prci
+
+  reg:
+    maxItems: 1
+
+  clocks:
+    items:
+      - description: high frequency clock.
+      - description: RTL clock.
+
+  clock-names:
+    items:
+      - const: hfclk
+      - const: rtcclk
+
+  "#clock-cells":
+    const: 1
+
+required:
+  - compatible
+  - reg
+  - clocks
+  - "#clock-cells"
+
+additionalProperties: false
+
+examples:
+  - |
+    prci: clock-controller@10000000 {
+      compatible = "sifive,fu740-c000-prci";
+      reg = <0x10000000 0x1000>;
+      clocks = <&hfclk>, <&rtcclk>;
+      #clock-cells = <1>;
+    };