dts: Add a binding for hid-over-i2c

Add this binding from Linux v5.4.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

doc/device-tree-bindings/input/hid-over-i2c.txt

@@ -0,0 +1,44 @@
+* HID over I2C Device-Tree bindings
+
+HID over I2C provides support for various Human Interface Devices over the
+I2C bus. These devices can be for example touchpads, keyboards, touch screens
+or sensors.
+
+The specification has been written by Microsoft and is currently available here:
+http://msdn.microsoft.com/en-us/library/windows/hardware/hh852380.aspx
+
+If this binding is used, the kernel module i2c-hid will handle the communication
+with the device and the generic hid core layer will handle the protocol.
+
+Required properties:
+- compatible: must be "hid-over-i2c"
+- reg: i2c slave address
+- hid-descr-addr: HID descriptor address
+- interrupts: interrupt line
+
+Additional optional properties:
+
+Some devices may support additional optional properties to help with, e.g.,
+power sequencing. The following properties can be supported by one or more
+device-specific compatible properties, which should be used in addition to the
+"hid-over-i2c" string.
+
+- compatible:
+  * "wacom,w9013" (Wacom W9013 digitizer). Supports:
+    - vdd-supply (3.3V)
+    - vddl-supply (1.8V)
+    - post-power-on-delay-ms
+
+- vdd-supply: phandle of the regulator that provides the supply voltage.
+- post-power-on-delay-ms: time required by the device after enabling its regulators
+  or powering it on, before it is ready for communication.
+
+Example:
+
+	i2c-hid-dev@2c {
+		compatible = "hid-over-i2c";
+		reg = <0x2c>;
+		hid-descr-addr = <0x0020>;
+		interrupt-parent = <&gpx3>;
+		interrupts = <3 2>;
+	};