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>