mfd: bd71837: Core driver for ROHM BD71837 PMIC

ROHM BD71837 PMIC MFD driver providing interrupts and support
for three subsystems:
- clk
- Regulators
- input/power-key

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>