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>
diff --git a/drivers/mfd/Kconfig b/drivers/mfd/Kconfig
index fc419da..82f70dc 100644
--- a/drivers/mfd/Kconfig
+++ b/drivers/mfd/Kconfig
@@ -597,6 +597,17 @@
 	  Additional drivers must be enabled in order to use the
 	  different functionality of the device.
 
+config MFD_RN5T618
+	tristate "Ricoh RN5T5618 PMIC"
+	depends on I2C
+	select MFD_CORE
+	select REGMAP_I2C
+	help
+	  Say yes here to add support for the Ricoh RN5T618 PMIC. This
+	  driver provides common support for accessing the device,
+	  additional drivers must be enabled in order to use the
+	  functionality of the device.
+
 config MFD_SEC_CORE
 	bool "SAMSUNG Electronics PMIC Series Support"
 	depends on I2C=y