regulator: bd71837: adobt MFD changes to regulator driver

MFD part for bd71837 was changed during the review. Clean regulator part to
match changed MFD:
- renamed header file => fix include
- remove unused platdata as also type definition was removed
- Kconfig option for MFD part was changed => fix depends on clause
- Rename Kconfig option for regulators
  As Kconfig option for regulators gets now used (when dependency to MFD is
  satisfied) change it so that it won't require new change when support for
  bd71847 is added.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Mark Brown <broonie@kernel.org>