regulator: act8945a-regulator: add shutdown function

Implement shutdown method to make sure the PMIC will not enter the suspend
state when the system is shutdown.

This work is based on work done by Borris Brezillon on [1].

[1] https://www.spinics.net/lists/kernel/msg2942960.html



Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Mark Brown <broonie@kernel.org>