mfd: Add TI TPS80031 mfd core driver

TPS80031/ TPS80032 Fully Integrated Power Management with Power
Path and Battery Charger. The device provides five configurable
step-down converters, 11 general purpose LDOs, USB OTG Module,
ADC, RTC, 2 PWM, System Voltage Regulator/Battery Charger with
Power Path from USB, 32K clock generator.

Add the mfd core driver for TPS80031/TPS80032.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviwed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>