arm64: dts: rockchip: add gpio-keys to Qnap-TS433

The TS433 has 3 buttons, power and copy in the front as well as a reset
pinhole button on the back. The power-button is connected to the embedded
controller while the other two buttons are just gpio connected.

Add the gpio-keys definition for the two buttons we can handle right now.

Tested-by: Uwe Kleine-König <ukleinek@debian.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20240723195538.1133436-11-heiko@sntech.de