regulator: rn5t618: add driver for Ricoh RN5T618 regulators

This driver supports the 3 DCDC and 7 LDO regulators available on
Ricoh RN5T618 PMIC.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>