dt-bindings: pwm: Add pwm-gpio

Add bindings for PWM modulated by GPIO.
Signed-off-by: Nicola Di Lieto <nicola.dilieto@gmail.com>
Co-developed-by: Stefan Wahren <wahrenst@gmx.net>
Signed-off-by: Stefan Wahren <wahrenst@gmx.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Link: https://lore.kernel.org/r/20240604-pwm-gpio-v7-1-6b67cf60db92@linaro.orgSigned-off-by: Uwe Kleine-König <ukleinek@kernel.org>

Documentation/devicetree/bindings/pwm/pwm-gpio.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/pwm/pwm-gpio.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Generic software PWM for modulating GPIOs
+
+maintainers:
+  - Stefan Wahren <wahrenst@gmx.net>
+
+allOf:
+  - $ref: pwm.yaml#
+
+properties:
+  compatible:
+    const: pwm-gpio
+
+  "#pwm-cells":
+    const: 3
+    description:
+      See pwm.yaml in this directory for a description of the cells format.
+      The first cell which represents the PWM instance number must always
+      be zero.
+
+  gpios:
+    description:
+      GPIO to be modulated
+    maxItems: 1
+
+required:
+  - compatible
+  - "#pwm-cells"
+  - gpios
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/gpio/gpio.h>
+
+    pwm {
+        #pwm-cells = <3>;
+        compatible = "pwm-gpio";
+        gpios = <&gpio 1 GPIO_ACTIVE_HIGH>;
+    };