dt-bindings: gpio: Document the Xylon LogiCVC GPIO controller

The Xylon LogiCVC display controller exports some GPIOs, which are
exposed as a separate entity.
Signed-off-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191203141243.251058-4-paul.kocialkowski@bootlin.comSigned-off-by: Linus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/gpio/xylon,logicvc-gpio.yaml

+# SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
+# Copyright 2019 Bootlin
+%YAML 1.2
+---
+$id: "http://devicetree.org/schemas/gpio/xylon,logicvc-gpio.yaml#"
+$schema: "http://devicetree.org/meta-schemas/core.yaml#"
+
+title: Xylon LogiCVC GPIO controller
+
+maintainers:
+  - Paul Kocialkowski <paul.kocialkowski@bootlin.com>
+
+description: |
+  The LogiCVC GPIO describes the GPIO block included in the LogiCVC display
+  controller. These are meant to be used for controlling display-related
+  signals.
+
+  The controller exposes GPIOs from the display and power control registers,
+  which are mapped by the driver as follows:
+  - GPIO[4:0] (display control) mapped to index 0-4
+  - EN_BLIGHT (power control) mapped to index 5
+  - EN_VDD (power control) mapped to index 6
+  - EN_VEE (power control) mapped to index 7
+  - V_EN (power control) mapped to index 8
+
+properties:
+  $nodename:
+    pattern: "^gpio@[0-9a-f]+$"
+
+  compatible:
+    enum:
+      - xylon,logicvc-3.02.a-gpio
+
+  reg:
+    maxItems: 1
+
+  "#gpio-cells":
+    const: 2
+
+  gpio-controller: true
+
+  gpio-line-names:
+    minItems: 1
+    maxItems: 9
+
+required:
+  - compatible
+  - reg
+  - "#gpio-cells"
+  - gpio-controller
+
+examples:
+  - |
+    logicvc: logicvc@43c00000 {
+      compatible = "xylon,logicvc-3.02.a", "syscon", "simple-mfd";
+      reg = <0x43c00000 0x6000>;
+
+      #address-cells = <1>;
+      #size-cells = <1>;
+
+      logicvc_gpio: gpio@40 {
+        compatible = "xylon,logicvc-3.02.a-gpio";
+        reg = <0x40 0x40>;
+        gpio-controller;
+        #gpio-cells = <2>;
+        gpio-line-names = "GPIO0", "GPIO1", "GPIO2", "GPIO3", "GPIO4",
+               "EN_BLIGHT", "EN_VDD", "EN_VEE", "V_EN";
+      };
+    };