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.info


Signed-off-by: Lee Jones <lee@kernel.org>