Each regulator_dev is locked with its own mutex. This is fine as long
as only one regulator_dev is locked, but makes lockdep unhappy when we
have to walk up the supply chain like it can happen in
regulator_get_voltage:

regulator_get_voltage ->
 mutex_lock(&regulator->rdev->mutex) ->
_regulator_get_voltage(regulator->rdev) ->
regulator_get_voltage(rdev->supply) ->
mutex_lock(&regulator->rdev->mutex);

This causes lockdep to issue a possible deadlock warning.

There are at least two ways to work around this:

- We can always lock the whole supply chain using the functions
  introduced with this patch.
- We could store the current voltage in struct regulator_rdev so
  that we do not have to walk up the supply chain for the
  _regulator_get_voltage case.

Anyway, regulator_lock_supply/regulator_unlock_supply will be needed
once we allow regulator_set_voltage to optimize the supply voltages.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Mark Brown <broonie@kernel.org>