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/arm/dts/rk3288-popmetal.dtsi b/arch/arm/dts/rk3288-popmetal.dtsi
index 63785eb..0253933 100644
--- a/arch/arm/dts/rk3288-popmetal.dtsi
+++ b/arch/arm/dts/rk3288-popmetal.dtsi
@@ -38,6 +38,7 @@
  *     OTHER DEALINGS IN THE SOFTWARE.
  */
 
+#include <dt-bindings/input/input.h>
 #include "rk3288.dtsi"
 
 / {
@@ -63,6 +64,7 @@
 		power {
 			gpios = <&gpio0 5 GPIO_ACTIVE_LOW>;
 			label = "GPIO Key Power";
+			linux,code = <KEY_POWER>;
 			linux,input-type = <1>;
 			wakeup-source;
 			debounce-interval = <100>;