dt-bindings: power: reset: atmel,sama5d2-shdwc: convert to yaml

Convert Microchip AT91 SAMA5D2 shutdown controller to YAML. SAMA7G5 SHDWC
DT node (available in arch/arm/boot/dts/sama7g5.dtsi) has syscon along with
its compatible. There is no usage of this syscon in the current code but it
may be necessary in future as some registers of SHDWC are accessed in
different drivers (at91-sama5d2_shdwc.c and arch/arm/mach-at91/pm.c).
Thus update the YAML with it to make DT checkers happy.

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