mfd: Add Ricoh RN5T618 PMIC core driver

Ricoh RN5T618 is a power management IC which integrates 3 step-down
DCDC converters, 7 low-dropout regulators, a Li-ion battery charger,
fuel gauge, ADC, GPIOs and a watchdog timer.

This commit adds a MFD core driver to support the I2C communication
with the device.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>