ARM: dts: at91: sama7g5: add shdwc node

Add shutdown controller node and enable it.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20210823131915.23857-5-claudiu.beznea@microchip.com