debugfs code complained at boot time that

  debugfs: Directory 'reg-dummy-regulator-dummy' with parent 'regulator'
  already present!

if we compile kernel with DEBUG_TEST_DRIVER_REMOVE. The problem is that we
don't provide .remove() method for dummy_regulator_driver, which should
invoke regulator_unregister() on device teardown to properly free things.

Though it's harmless as dummy_pdev never gets unbound in practice, let's
use devm_regulator_register() to get rid of the inconsistency.
Signed-off-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20210925035507.1904-1-yuzenghui@huawei.comSigned-off-by: Mark Brown <broonie@kernel.org>