mfd: Add TPS6586x driver

Add mfd core driver for TPS6586x PMICs family.
The driver provides I/O access for the sub-device drivers and performs
regstration of the sub-devices based on the platform requirements.
In addition it implements GPIOlib interface for the chip GPIOs.

TODO:
        - add interrupt support
        - add platform data for PWM, backlight leds and charger

Signed-off-by: Mike Rapoport <mike@compulab.co.il>
Signed-off-by: Mike Rapoport <mike.rapoport@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>