dts: add missing linux,code in gpio-keys

gpio-keys linux driver enforces user to specify linux,code.
Add missing linux,code before implementing button input support.

- arch/arm/dts/rk3288-popmetal.dtsi -> KEY_POWER
- arch/arm/dts/rk3288-tinker.dtsi -> KEY_POWER
- arch/arm/dts/am3517-evm-ui.dtsi -> KEY_RECORD
- sandbox/dts/sandbox.dtsi -> BTN_1
- sandbox/dts/sandbox.dts -> BTN_1

Signed-off-by: Dzmitry Sankouski <dsankouski@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/arch/sandbox/dts/sandbox.dtsi b/arch/sandbox/dts/sandbox.dtsi
index 18bf1cb..7e7fcff 100644
--- a/arch/sandbox/dts/sandbox.dtsi
+++ b/arch/sandbox/dts/sandbox.dtsi
@@ -4,6 +4,8 @@
  * and sandbox64 builds.
  */
 
+#include <dt-bindings/input/input.h>
+
 #define USB_CLASS_HUB			9
 
 / {
@@ -36,11 +38,13 @@
 		btn1 {
 			gpios = <&gpio_a 3 0>;
 			label = "button1";
+		    linux,code = <BTN_1>;
 		};
 
 		btn2 {
 			gpios = <&gpio_a 4 0>;
 			label = "button2";
+		    linux,code = <BTN_2>;
 		};
 	};