regulator: pfuze100-regulator: provide pm_power_off_prepare handler

On some boards the SoC can use one pin "PMIC_STBY_REQ" to notify th PMIC
about state changes. In this case internal state of PMIC must be
preconfigured for upcomming state change.
It works fine with the current regulator framework, except with the
power-off case.

This patch is providing an optional pm_power_off_prepare handler
which will configure standby state of the PMIC to disable all power lines.

In my power consumption test on RIoTBoard, I got the following results:
power off without this patch:	320 mA
power off with this patch:	2   mA
suspend to ram:			40  mA

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