regulator: core: allow regulator_register() with

Merge series from André Draszik <andre.draszik@linaro.org>:

With these attached patches it becomes possible again to support
hardware designs with multiple PMICs where individual rails of each act
as required supplies for rails of the other (due to the latter being
e.g. always-on), and vice-versa.

Google Pixel 6 and 6 Pro (oriole and raven) are examples of such
designs.

Rather than returning -EPORBE_DEFER in regulator_register() when
set_machine_constraints() fails with -EPROBE_DEFER (due to missing
required supplies), we still allow rail registration and try to
reresolve supplies each time a new rail gets registered.

This is implemented using a bus (regulator bus), which allows the core
to reresolve supplies for regulators that still need them whenever new
regulators (i.e. devices) are added.

Using a bus also solves existing problems around late resolution of
supplies as mentioned in the commit message introducing that bus.

The series starts with a few bug fixes and the last two commits
implement the changes mentioned above, but do depend on the bug fixes.