dt-bindings: mfd: ti,twl: Document system-power-controller

Add system-power-controller property because these chips
can power off the device.
Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240217082007.3238948-2-andreas@kemnade.infoSigned-off-by: Lee Jones <lee@kernel.org>

Documentation/devicetree/bindings/mfd/ti,twl.yaml

@@ -34,6 +34,8 @@ properties:
 
   interrupt-controller: true
 
+  system-power-controller: true
+
   "#interrupt-cells":
     const: 1