mfd: rt4831: Adds DT binding document for Richtek RT4831

Adds DT binding document for Richtek RT4831.
Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/richtek,rt4831.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/mfd/richtek,rt4831.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Richtek RT4831 DSV and Backlight Integrated IC
+
+maintainers:
+  - ChiYuan Huang <cy_huang@richtek.com>
+
+description: |
+  RT4831 is a multifunctional device that can provide power to the LCD display
+  and LCD backlight.
+
+  For Display Bias Voltage DSVP and DSVN, the output range is about 4V to 6.5V.
+  It's sufficient to meet the current LCD power requirement.
+
+  For the LCD backlight, it can provide four channel WLED driving capability.
+  Each channel driving current is up to 30mA
+
+  Datasheet is available at
+  https://www.richtek.com/assets/product_file/RT4831A/DS4831A-05.pdf
+
+properties:
+  compatible:
+    const: richtek,rt4831
+
+  reg:
+    description: I2C device address.
+    maxItems: 1
+
+  enable-gpios:
+    description: |
+      GPIO to enable/disable the chip. It is optional.
+      Some usage directly tied this pin to follow VIO 1.8V power on sequence.
+    maxItems: 1
+
+  regulators:
+    $ref: ../regulator/richtek,rt4831-regulator.yaml
+
+  backlight:
+    $ref: ../leds/backlight/richtek,rt4831-backlight.yaml
+
+required:
+  - compatible
+  - reg
+
+additionalProperties: false
+
+examples:
+  - |
+    #include <dt-bindings/leds/rt4831-backlight.h>
+    i2c {
+      #address-cells = <1>;
+      #size-cells = <0>;
+
+      rt4831@11 {
+        compatible = "richtek,rt4831";
+        reg = <0x11>;
+
+        regulators {
+          DSVLCM {
+            regulator-min-microvolt = <4000000>;
+            regulator-max-microvolt = <7150000>;
+            regulator-allow-bypass;
+          };
+          DSVP {
+            regulator-name = "rt4831-dsvp";
+            regulator-min-microvolt = <4000000>;
+            regulator-max-microvolt = <6500000>;
+            regulator-boot-on;
+          };
+          DSVN {
+            regulator-name = "rt4831-dsvn";
+            regulator-min-microvolt = <4000000>;
+            regulator-max-microvolt = <6500000>;
+            regulator-boot-on;
+          };
+        };
+
+        backlight {
+          compatible = "richtek,rt4831-backlight";
+          default-brightness = <1024>;
+          max-brightness = <2048>;
+          richtek,bled-ovp-sel = /bits/ 8 <RT4831_BLOVPLVL_21V>;
+          richtek,channel-use = /bits/ 8 <RT4831_BLED_ALLCHEN>;
+        };
+      };
+    };