ARM: dts: at91: use generic name for shutdown controller

Use poweroff generic name for shdwc node to cope with device tree
specifications.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20230616101646.879480-2-claudiu.beznea@microchip.com