regulator: bd718x7: Remove struct bd718xx_pmic

All the fields in struct bd718xx_pmic are not really necessary.
Remove struct bd718xx_pmic to simplify the code.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Reviewed-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Mark Brown <broonie@kernel.org>