regulator: bd718x7: Split driver to common and bd718x7 specific parts

Few ROHM PMICs allow setting the voltage states for different system states
like RUN, IDLE, SUSPEND and LPSR. States are then changed via SoC specific
mechanisms. bd718x7 driver implemented device-tree parsing functions for
these state specific voltages. The parsing functions can be re-used by
other ROHM chip drivers like bd71828. Split the generic functions from
bd718x7-regulator.c to rohm-regulator.c and export them for other modules
to use.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>