mfd: bd71828: Support ROHM BD71828 PMIC - core

BD71828GW is a single-chip power management IC for battery-powered portable
devices. The IC integrates 7 buck converters, 7 LDOs, and a 1500 mA
single-cell linear charger. Also included is a Coulomb counter, a real-time
clock (RTC), 3 GPO/regulator control pins, HALL input and a 32.768 kHz
clock gate.

Add MFD core driver providing interrupt controller facilities and i2c
access to sub device drivers.

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