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

[ Upstream commit 327ca228

 ]

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
Stable-dep-of: f6ad3c13

 ("ARM: dts: at91: sam9x60: fix the SOC detection")
Signed-off-by: Sasha Levin <sashal@kernel.org>