regulator: mc13xxx: Fix probing with DT

The nodes of regulators should be retrieved from parent device.
Bug was be introduced by commit (regulator: mc13xxx: Fix NULL
pointer error in non-DT mode) in conjuction with (mfd: Revert
"mfd: Always assign of_node in mfd_add_device()").

Signed-off-by: Alexander Shiyan <shc_work@mail.ru>
Signed-off-by: Mark Brown <broonie@linaro.org>