regulator: da903x regulator bug fix

Changes the device registration part of the probe function to supply the
regulator device rather than its parent (the mfd device) as this caused
problems when the regulator core attempted to find constraints associated
with the regulators.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Acked-by: Eric Miao <eric.miao@marvell.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>