mfd: Add Maxim 77686 driver

This patch is device driver for MAX77686 chip.
MAX77686 is PMIC and includes regulator and rtc on it.
This driver is core of MAX77686 chip, so provides common support for
accessing on-chip devices. It uses irq_domain to manage irq and regmap
to read/write data to its register with i2c bus.

Signed-off-by: Chiwoong Byun <woong.byun@samsung.com>
Signed-off-by: Jonghwa Lee <jonghwa3.lee@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>