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