regulator: bd71828: Basic support for ROHM bd71828 PMIC regulators

ROHM BD71828 is a power management IC containing 7 bucks and 7 LDOs. Bucks
1,2,6 and 7 can be assigned to a regulator group controlled by run-levels.
Eg. Voltages and enable/disable statuses for specific run-levels (run0 to
run3) can be set via register interface and run level changes can then be
done either via I2C or GPIO.

This initial commit does not support assigning bucks to be controlled via
run-levels but only allows them to be individually controlled.

LDO5 voltage can also be controlled by GPIO2 pin and register interfaces
but this driver only supports the control via register.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Link: https://lore.kernel.org/r/5b1c4a22c7945e97ff2a7924abfeb3239043f8eb.1576054779.git.matti.vaittinen@fi.rohmeurope.com
Signed-off-by: Mark Brown <broonie@kernel.org>