regulator: pfuze100: add fsl,pmic-stby-poweroff property

Document the new optional "fsl,pmic-stby-poweroff" property.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>