dt-bindings: atmel-sysreg: add microchip,sama7g5-shdwc

Add compatible for Microchip SAMA7G5's shutdown controller.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>