Support voltage control for regulators on ROHM BD71815 PMIC.

ROHM BD71815 contains 5 bucks, 7 LDOs and a boost (intended for LED).
Bucks 1 and 2 support HW state based voltage level and enable states. Other
regulators support HW state based enable states. All bucks and LDOs 1-5
allow voltage changes for RUN state and LDO4 can be enabled/disabled via
GPIO.

LDO3 does support changing between two predetermined voltages by using
a GPIO but this functionality is not included in this commit.

This work is derived from driver originally written by Tony Luo
<luofc@embedinfo.com> - although not much of original work is left.
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>