regulator: rn5t618: Convert to new style DT parsing

Use the simplified DT parsing method to remove some duplicated
code.

Since this is a MFD subdevice and its device object doesn't have an
associated DT node, the configuration instance used to register the
regulators has been changed to point to the parent device.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>