dt-bindings: input: adc-keys bindings documentation

Dump adc-keys bindings documentation from Linux kernel source tree from
commit 698dc0cf9447 ("dt-bindings: input: adc-keys: clarify
description").

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>

doc/device-tree-bindings/input/adc-keys.txt

+ADC attached resistor ladder buttons
+------------------------------------
+
+Required properties:
+ - compatible: "adc-keys"
+ - io-channels: Phandle to an ADC channel
+ - io-channel-names = "buttons";
+ - keyup-threshold-microvolt: Voltage above or equal to which all the keys are
+			      considered up.
+
+Optional properties:
+	- poll-interval: Poll interval time in milliseconds
+	- autorepeat: Boolean, Enable auto repeat feature of Linux input
+	  subsystem.
+
+Each button (key) is represented as a sub-node of "adc-keys":
+
+Required subnode-properties:
+	- label: Descriptive name of the key.
+	- linux,code: Keycode to emit.
+	- press-threshold-microvolt: voltage above or equal to which this key is
+				     considered pressed.
+
+No two values of press-threshold-microvolt may be the same.
+All values of press-threshold-microvolt must be less than
+keyup-threshold-microvolt.
+
+Example:
+
+#include <dt-bindings/input/input.h>
+
+	adc-keys {
+		compatible = "adc-keys";
+		io-channels = <&lradc 0>;
+		io-channel-names = "buttons";
+		keyup-threshold-microvolt = <2000000>;
+
+		button-up {
+			label = "Volume Up";
+			linux,code = <KEY_VOLUMEUP>;
+			press-threshold-microvolt = <1500000>;
+		};
+
+		button-down {
+			label = "Volume Down";
+			linux,code = <KEY_VOLUMEDOWN>;
+			press-threshold-microvolt = <1000000>;
+		};
+
+		button-enter {
+			label = "Enter";
+			linux,code = <KEY_ENTER>;
+			press-threshold-microvolt = <500000>;
+		};
+	};
+
++--------------------------------+------------------------+
+| 2.000.000 <= value             | no key pressed         |
++--------------------------------+------------------------+
+| 1.500.000 <= value < 2.000.000 | KEY_VOLUMEUP pressed   |
++--------------------------------+------------------------+
+| 1.000.000 <= value < 1.500.000 | KEY_VOLUMEDOWN pressed |
++--------------------------------+------------------------+
+|   500.000 <= value < 1.000.000 | KEY_ENTER pressed      |
++--------------------------------+------------------------+
+|              value <   500.000 | no key pressed         |
++--------------------------------+------------------------+