ARM: dts: rockchip: enable adc key for rk3288-evb

rk3288-evb have many tablet-used adc buttons.
Signed-off-by: NJacob Chen <jacob-chen@iotwrt.com>
Signed-off-by: NHeiko Stuebner <heiko@sntech.de>

arch/arm/boot/dts/rk3288-evb.dtsi

@@ -48,6 +48,43 @@
 		reg = <0x0 0x80000000>;
 	};
 
+	adc-keys {
+		compatible = "adc-keys";
+		io-channels = <&saradc 1>;
+		io-channel-names = "buttons";
+		keyup-threshold-microvolt = <1800000>;
+
+		button-up {
+			label = "Volume Up";
+			linux,code = <KEY_VOLUMEUP>;
+			press-threshold-microvolt = <100000>;
+		};
+
+		button-down {
+			label = "Volume Down";
+			linux,code = <KEY_VOLUMEDOWN>;
+			press-threshold-microvolt = <300000>;
+		};
+
+		menu {
+			label = "Menu";
+			linux,code = <KEY_MENU>;
+			press-threshold-microvolt = <640000>;
+		};
+
+		esc {
+			label = "Esc";
+			linux,code = <KEY_ESC>;
+			press-threshold-microvolt = <1000000>;
+		};
+
+		home  {
+			label = "Home";
+			linux,code = <KEY_HOME>;
+			press-threshold-microvolt = <1300000>;
+		};
+	};
+
 	backlight: backlight {
 		compatible = "pwm-backlight";
 		brightness-levels = <