ARM: dts: exynos: Add power button for Odroid XU3/4

The power button (SW2) on Odroid XU3/4 is connected to the PWRON pin
of the S2MPS11 PMIC.

The S2MPS11 datasheet says that ONOB pin operates as 'PWRON key active
low signal'. In fact, S2MPS11 PMIC acts as a 16ms debouce filter and
signal inverter, thus effectively repeating PWRON (active high) to ONOB
pin (active low).

ONOB PMIC pin is then connected to XEINT3 SoC pin, so we get the state
of the power button on the gpx0-3 GPIO.

This patch adds device-tree bindings for the power button of Odroid
XU3/4 boards.
Signed-off-by: NBrian Kim <brian.kim@hardkernel.com>
[mszyprow: extended commit message, added comments and fixed minor
 issues in the dts]
Signed-off-by: NMarek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: NAnand Moon <linux.amoon@gmail.com>
Tested-by: NAnand Moon <linux.amoon@gmail.com>
Signed-off-by: NKrzysztof Kozlowski <krzk@kernel.org>

arch/arm/boot/dts/exynos5422-odroidxu3-common.dtsi

@@ -13,6 +13,7 @@
 */
 
 #include <dt-bindings/clock/samsung,s2mps11.h>
+#include <dt-bindings/input/input.h>
 #include <dt-bindings/interrupt-controller/irq.h>
 #include <dt-bindings/gpio/gpio.h>
 #include <dt-bindings/sound/samsung-i2s.h>
@@ -41,6 +42,27 @@
 		};
 	};
 
+	gpio_keys {
+		compatible = "gpio-keys";
+		pinctrl-names = "default";
+		pinctrl-0 = <&power_key>;
+
+		power_key {
+			/*
+			 * The power button (SW2) is connected to the PWRON
+			 * pin (active high) of the S2MPS11 PMIC, which acts
+			 * as a 16ms debouce filter and signal inverter with
+			 * output on ONOB pin (active low). ONOB PMIC pin is
+			 * then connected to XEINT3 SoC pin.
+			 */
+			gpios = <&gpx0 3 GPIO_ACTIVE_LOW>;
+			linux,code = <KEY_POWER>;
+			label = "power key";
+			debounce-interval = <0>;
+			wakeup-source;
+		};
+	};
+
 	emmc_pwrseq: pwrseq {
 		pinctrl-0 = <&emmc_nrst_pin>;
 		pinctrl-names = "default";
@@ -760,6 +782,13 @@
 };
 
 &pinctrl_0 {
+	power_key: power-key {
+		samsung,pins = "gpx0-3";
+		samsung,pin-function = <EXYNOS_PIN_FUNC_INPUT>;
+		samsung,pin-pud = <EXYNOS_PIN_PULL_NONE>;
+		samsung,pin-drv = <EXYNOS5420_PIN_DRV_LV1>;
+	};
+
 	hdmi_hpd_irq: hdmi-hpd-irq {
 		samsung,pins = "gpx3-7";
 		samsung,pin-function = <EXYNOS_PIN_FUNC_INPUT>;