BD718x7 has a HW 'feature' which leaves regulators with SEL-bit
enabled (SW-control) disabled when transitioning from poweroff
to SNVS state. In other words - if any critical regulator has
been controlled by SW (enable/disable control) - it will stay
OFF if we go to SNVS state. SNVS is usually a nice idea with
i.MX8. So this change is required before we can enable SNVS
as a state to go after reset.

If SW control for crucial regulators is needed - then we
must not use SNVS. Sorry.

In default setup this means that all other regulators except
buck3 and buck4 (GPU and VDU power) on BD71837 must not be
controlled by SW.

Buck 1 and 2 support DVS - Eg voltage can be changed when buck
is enabled. So voltage change can be supported for those.
As said - BD71837 buck 3 and 4 aren't crucial for boot so we
can provide full control to those - if HW leaves them shut down
after SNVS transition then linux can enable them again at boot.*

*NOTE: bd718x7 driver is _not_ enabling/disabling buck3/buck4
in suspend/resume callbacks. If this is wanted we can implement
it in this driver or leave it for driver which uses the regulator
APIs and controls buck states for other power saving use-cases
(galcore that is?).

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>