regulator: tps65218: Enable suspend configuration

TPS65218 has a pre-defined power-up / power-down sequence which in
a typical application does not need to be changed. However, it is possible
to define custom sequences under I2C control. The power-up sequence is
defined by strobes and delay times. Each output rail is assigned to a
strobe to determine the order in which the rails are enabled.

Every regulator has sequence registers and every regulator has a default
strobe value and gets disabled when a particular power down sequence
occurs.

To keep a regulator on during suspend we write value 0 to strobe so
that the regulator is out of all sequencers and is not impacted by any
power down sequence. Hence saving the default strobe value during probe
so that when we want to regulator to be enabled during suspend we write 0
to strobe and when we want it to get disabled during suspend we write
the default saved strobe value.
This allows platform data to specify which power rails should be on or off
during RTC only suspend. This is necessary to keep DDR state while in RTC
only suspend.

Signed-off-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>