ARM: dts: at91: sama5d4_xplained: change the key code of the gpio key

Having a button code and not a key code causes issues with libinput.
udev won't set ID_INPUT_KEY. If it is forced, then it causes a bug
within libinput.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210727091351.44475-1-nicolas.ferre@microchip.com