backlight: rt4831: Adds DT binding document for Richtek RT4831 backlight

Adds DT binding document for Richtek RT4831 backlight.
Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/leds/backlight/richtek,rt4831-backlight.yaml

+# SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause)
+%YAML 1.2
+---
+$id: http://devicetree.org/schemas/leds/backlight/richtek,rt4831-backlight.yaml#
+$schema: http://devicetree.org/meta-schemas/core.yaml#
+
+title: Richtek RT4831 Backlight
+
+maintainers:
+  - ChiYuan Huang <cy_huang@richtek.com>
+
+description: |
+  RT4831 is a mutifunctional device that can provide power to the LCD display
+  and LCD backlight.
+
+  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
+
+allOf:
+  - $ref: common.yaml#
+
+properties:
+  compatible:
+    const: richtek,rt4831-backlight
+
+  default-brightness:
+    minimum: 0
+    maximum: 2048
+
+  max-brightness:
+    minimum: 0
+    maximum: 2048
+
+  richtek,pwm-enable:
+    description: |
+      Specify the backlight dimming following by PWM duty or by SW control.
+    type: boolean
+
+  richtek,bled-ovp-sel:
+    description: |
+      Backlight OVP level selection, currently support 17V/21V/25V/29V.
+    $ref: /schemas/types.yaml#/definitions/uint8
+    default: 1
+    minimum: 0
+    maximum: 3
+
+  richtek,channel-use:
+    description: |
+      Backlight LED channel to be used.
+      BIT 0/1/2/3 is used to indicate led channel 1/2/3/4 enable or disable.
+    $ref: /schemas/types.yaml#/definitions/uint8
+    minimum: 1
+    maximum: 15
+
+required:
+  - compatible
+  - richtek,channel-use
+
+additionalProperties: false

include/dt-bindings/leds/rt4831-backlight.h

+/* SPDX-License-Identifier: (GPL-2.0-only OR BSD-2-Clause) */
+/*
+ * This header provides constants for rt4831 backlight bindings.
+ *
+ * Copyright (C) 2020, Richtek Technology Corp.
+ * Author: ChiYuan Huang <cy_huang@richtek.com>
+ */
+
+#ifndef _DT_BINDINGS_RT4831_BACKLIGHT_H
+#define _DT_BINDINGS_RT4831_BACKLIGHT_H
+
+#define RT4831_BLOVPLVL_17V	0
+#define RT4831_BLOVPLVL_21V	1
+#define RT4831_BLOVPLVL_25V	2
+#define RT4831_BLOVPLVL_29V	3
+
+#define RT4831_BLED_CH1EN	(1 << 0)
+#define RT4831_BLED_CH2EN	(1 << 1)
+#define RT4831_BLED_CH3EN	(1 << 2)
+#define RT4831_BLED_CH4EN	(1 << 3)
+#define RT4831_BLED_ALLCHEN	((1 << 4) - 1)
+
+#endif /* _DT_BINDINGS_RT4831_BACKLIGHT_H */