dt-bindings: pwm: Document Synopsys DesignWare snps,pwm-dw-apb-timers-pwm2

Add documentation for the bindings for Synopsys' DesignWare PWM block
as we will be adding DT/platform support to the Linux driver soon.
Signed-off-by: Ben Dooks <ben.dooks@sifive.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Documentation/devicetree/bindings/pwm/snps,dw-apb-timers-pwm2.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+# Copyright (C) 2022 SiFive, Inc.
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/pwm/snps,dw-apb-timers-pwm2.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Synopsys DW-APB timers PWM controller
+
+maintainers:
+  - Ben Dooks <ben.dooks@sifive.com>
+
+description:
+  This describes the DesignWare APB timers module when used in the PWM
+  mode. The IP core can be generated with various options which can
+  control the functionality, the number of PWMs available and other
+  internal controls the designer requires.
+
+  The IP block has a version register so this can be used for detection
+  instead of having to encode the IP version number in the device tree
+  comaptible.
+
+allOf:
+  - $ref: pwm.yaml#
+
+properties:
+  compatible:
+    const: snps,dw-apb-timers-pwm2
+
+  reg:
+    maxItems: 1
+
+  "#pwm-cells":
+    const: 3
+
+  clocks:
+    items:
+      - description: Interface bus clock
+      - description: PWM reference clock
+
+  clock-names:
+    items:
+      - const: bus
+      - const: timer
+
+  snps,pwm-number:
+    $ref: /schemas/types.yaml#/definitions/uint32
+    description: The number of PWM channels configured for this instance
+    enum: [1, 2, 3, 4, 5, 6, 7, 8]
+
+required:
+  - compatible
+  - reg
+  - "#pwm-cells"
+  - clocks
+  - clock-names
+
+additionalProperties: false
+
+examples:
+  - |
+    pwm: pwm@180000 {
+      compatible = "snps,dw-apb-timers-pwm2";
+      reg = <0x180000 0x200>;
+      #pwm-cells = <3>;
+      clocks = <&bus>, <&timer>;
+      clock-names = "bus", "timer";
+    };