gpio/sifive: Add DT documentation for SiFive GPIO

# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)

%YAML 1.2

---

$id: http://devicetree.org/schemas/gpio/sifive,gpio.yaml#

$schema: http://devicetree.org/meta-schemas/core.yaml#

title: SiFive GPIO controller

maintainers:

  - Yash Shah <yash.shah@sifive.com>

  - Paul Walmsley <paul.walmsley@sifive.com>

properties:

  compatible:

    items:

      - const: sifive,fu540-c000-gpio

      - const: sifive,gpio0

  reg:

    maxItems: 1

  interrupts:

    description:

      interrupt mapping one per GPIO. Maximum 16 GPIOs.

    minItems: 1

    maxItems: 16

  interrupt-controller: true

  "#interrupt-cells":

    const: 2

  clocks:

    maxItems: 1

  "#gpio-cells":

    const: 2

  gpio-controller: true

required:

  - compatible

  - reg

  - interrupts

  - interrupt-controller

  - "#interrupt-cells"

  - clocks

  - "#gpio-cells"

  - gpio-controller

additionalProperties: false

examples:

  - |

      #include <dt-bindings/clock/sifive-fu540-prci.h>

      gpio@10060000 {

        compatible = "sifive,fu540-c000-gpio", "sifive,gpio0";

        interrupt-parent = <&plic>;

        interrupts = <7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22>;

        reg = <0x0 0x10060000 0x0 0x1000>;

        clocks = <&tlclk PRCI_CLK_TLCLK>;

        gpio-controller;

        #gpio-cells = <2>;

        interrupt-controller;

        #interrupt-cells = <2>;

      };

...