mfd: Support for ROHM BD71815 PMIC core

Add core support for ROHM BD71815 Power Management IC.

The IC integrates regulators, a battery charger with a coulomb counter,
a real-time clock (RTC), clock gate and general-purpose outputs (GPO).

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