MAINTAINERS: add documentation file for Microchip SAMA5D2 shutdown controller

Add documentation file for SAMA5D2 shutdown controller.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>