regulator: core: Add early supply resolution for regulators

The call to set_machine_constraints() in regulator_register(), will
attempt to get the voltage for the regulator. If a regulator is in
bypass will fail to get the voltage (ie. it's bypass voltage) and
hence register the regulator, because the supply for the regulator has
not been resolved yet.

To fix this, add a call to regulator_resolve_supply() before we call
set_machine_constraints(). If the call to regulator_resolve_supply()
fails, rather than returning an error at this point, allow the
registration of the regulator to continue because for some regulators
resolving the supply at this point may not be necessary and it will be
resolved later as more regulators are added. Furthermore, if the supply
is still not resolved for a bypassed regulator, this will be detected
when we attempt to get the voltage for the regulator and an error will
be propagated at this point.

If a bypassed regulator does not have a supply when we attempt to get
the voltage, rather than returing -EINVAL, return -EPROBE_DEFER instead
to allow the registration of the regulator to be deferred and tried
again later.

Please note that regulator_resolve_supply() will call
regulator_dev_lookup() which may acquire the regulator_list_mutex. To
avoid any deadlocks we cannot hold the regulator_list_mutex when calling
regulator_resolve_supply(). Therefore, rather than holding the lock
around a large portion of the registration code, just hold the lock when
aquiring any GPIOs and setting up supplies because these sections may
add entries to the regulator_map_list and regulator_ena_gpio_list,
respectively.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Mark Brown <broonie@kernel.org>