boards: arm: atsamd21: enable button in dts

The SAM D21 Xplained Pro has a button connected to PA15, so enable it
in the dts.

Signed-off-by: Benjamin Valentin <[email protected]>

Author: benpicco

boards/arm/atsamd21_xpro/atsamd21_xpro.dts

@@ -21,6 +21,7 @@
 	/* These aliases are provided for compatibility with samples */
 	aliases {
 		led0 = &led0;
+		sw0 = &user_button;
 	};
 
 	leds {
@@ -30,6 +31,14 @@
 			label = "Yellow LED";
 		};
 	};
+
+	buttons {
+		compatible = "gpio-keys";
+		user_button: button_0 {
+			gpios = <&porta 15 (GPIO_PUD_PULL_UP | GPIO_INT_ACTIVE_LOW)>;
+			label = "SW0";
+		};
+	};
 };
 
 &sercom0 {