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/imx6ul-phytec-segin-peb-eval-01.dtsi b/arch/arm/dts/imx6ul-phytec-segin-peb-eval-01.dtsi
index 2f3fd32..5f760ed 100644
--- a/arch/arm/dts/imx6ul-phytec-segin-peb-eval-01.dtsi
+++ b/arch/arm/dts/imx6ul-phytec-segin-peb-eval-01.dtsi
@@ -8,7 +8,7 @@
 
 / {
 	gpio_keys: gpio-keys {
-		compatible = "gpio-key";
+		compatible = "gpio-keys";
 		pinctrl-names = "default";
 		pinctrl-0 = <&pinctrl_gpio_keys>;
 		status = "disabled";