ARM: dts: at91: use the right property for shutdown controller

The wrong property "atmel,shdwc-debouncer" was used to specify the
debounce delay for the shutdown controler. Replace it with the
documented and implemented property "debounce-delay-us", as mentioned
in v4 driver submission. See:
https://lore.kernel.org/r/1458134390-23847-3-git-send-email-nicolas.ferre@atmel.com/



Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Reported-by: Clément Léger <clement.leger@bootlin.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20210730172729.28093-1-nicolas.ferre@microchip.com/