dt-bindings: mfd: as3722: Drop reference to pm_power_off

Devicetree bindings are supposed to be operating system independent and
should thus not describe how a specific functionality is implemented in
Linux.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Rob Herring <robh@kernel.org>