dm: button: add support for linux_code in button-gpio.c driver

Linux event code must be used in input devices, using buttons.

Signed-off-by: Dzmitry Sankouski <dsankouski@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/test.dts b/arch/sandbox/dts/test.dts
index 716e8c7..88d4d3c 100644
--- a/arch/sandbox/dts/test.dts
+++ b/arch/sandbox/dts/test.dts
@@ -172,11 +172,13 @@
 		btn1 {
 			gpios = <&gpio_a 3 0>;
 			label = "button1";
+			linux,code = <BTN_1>;
 		};
 
 		btn2 {
 			gpios = <&gpio_a 4 0>;
 			label = "button2";
+			linux,code = <BTN_2>;
 		};
 	};