dt-bindings: backlight: Add MPS MP3309C

The Monolithic Power (MPS) MP3309C is a WLED step-up converter, featuring a
programmable switching frequency to optimize efficiency.
The brightness can be controlled either by I2C commands (called "analog"
mode) or by a PWM input signal (PWM mode).
This driver supports both modes.

For device driver details, please refer to:
- drivers/video/backlight/mp3309c_bl.c

The datasheet is available at:
- https://www.monolithicpower.com/en/mp3309c.htmlSigned-off-by: Flavio Suligoi <f.suligoi@asem.it>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230925122609.78849-1-f.suligoi@asem.itSigned-off-by: Lee Jones <lee@kernel.org>

Documentation/devicetree/bindings/leds/backlight/mps,mp3309c.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/leds/backlight/mps,mp3309c.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: MPS MP3309C backlight
+
+maintainers:
+  - Flavio Suligoi <f.suligoi@asem.it>
+
+description: |
+  The Monolithic Power (MPS) MP3309C is a WLED step-up converter, featuring a
+  programmable switching frequency to optimize efficiency.
+  It supports two different dimming modes:
+
+  - analog mode, via I2C commands (default)
+  - PWM controlled mode.
+
+  The datasheet is available at:
+  https://www.monolithicpower.com/en/mp3309c.html
+
+allOf:
+  - $ref: common.yaml#
+
+properties:
+  compatible:
+    const: mps,mp3309c
+
+  reg:
+    maxItems: 1
+
+  pwms:
+    description: if present, the backlight is controlled in PWM mode.
+    maxItems: 1
+
+  enable-gpios:
+    description: GPIO used to enable the backlight in "analog-i2c" dimming mode.
+    maxItems: 1
+
+  mps,overvoltage-protection-microvolt:
+    description: Overvoltage protection (13.5V, 24V or 35.5V).
+    enum: [ 13500000, 24000000, 35500000 ]
+    default: 35500000
+
+  mps,no-sync-mode:
+    description: disable synchronous rectification mode
+    type: boolean
+
+required:
+  - compatible
+  - reg
+  - max-brightness
+  - default-brightness
+
+unevaluatedProperties: false
+
+examples:
+  - |
+    i2c {
+        #address-cells = <1>;
+        #size-cells = <0>;
+
+        /* Backlight with PWM control */
+        backlight_pwm: backlight@17 {
+            compatible = "mps,mp3309c";
+            reg = <0x17>;
+            pwms = <&pwm1 0 3333333 0>; /* 300 Hz --> (1/f) * 1*10^9 */
+            max-brightness = <100>;
+            default-brightness = <80>;
+            mps,overvoltage-protection-microvolt = <24000000>;
+        };
+    };