ARM64: dts: meson-gxbb-p200: add ADC laddered keys

Add the 5 buttons connected to a resistor laddered matrix and sampled
by the SAR ADC channel 0.

Only the p200 board has these buttons, the P201 doesn't.
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>

arch/arm64/boot/dts/amlogic/meson-gxbb-p200.dts

@@ -45,10 +45,55 @@
 /dts-v1/;
 
 #include "meson-gxbb-p20x.dtsi"
+#include <dt-bindings/input/input.h>
 
 / {
 	compatible = "amlogic,p200", "amlogic,meson-gxbb";
 	model = "Amlogic Meson GXBB P200 Development Board";
+
+	avdd18_usb_adc: regulator-avdd18_usb_adc {
+		compatible = "regulator-fixed";
+		regulator-name = "AVDD18_USB_ADC";
+		regulator-min-microvolt = <1800000>;
+		regulator-max-microvolt = <1800000>;
+	};
+
+	adc_keys {
+		compatible = "adc-keys";
+		io-channels = <&saradc 0>;
+		io-channel-names = "buttons";
+		keyup-threshold-microvolt = <1800000>;
+
+		button-home {
+			label = "Home";
+			linux,code = <KEY_HOME>;
+			press-threshold-microvolt = <900000>; /* 50% */
+		};
+
+		button-esc {
+			label = "Esc";
+			linux,code = <KEY_ESC>;
+			press-threshold-microvolt = <684000>; /* 38% */
+		};
+
+		button-up {
+			label = "Volume Up";
+			linux,code = <KEY_VOLUMEUP>;
+			press-threshold-microvolt = <468000>; /* 26% */
+		};
+
+		button-down {
+			label = "Volume Down";
+			linux,code = <KEY_VOLUMEDOWN>;
+			press-threshold-microvolt = <252000>; /* 14% */
+		};
+
+		button-menu {
+			label = "Menu";
+			linux,code = <KEY_MENU>;
+			press-threshold-microvolt = <0>; /* 0% */
+		};
+	};
 };
 
 &i2c_B {
@@ -56,3 +101,8 @@ &i2c_B {
 	pinctrl-0 = <&i2c_b_pins>;
 	pinctrl-names = "default";
 };
+
+&saradc {
+	status = "okay";
+	vref-supply = <&avdd18_usb_adc>;
+};