mfd: Add lp8788 mfd driver

TI LP8788 PMU provides regulators, battery charger, ADC,
RTC, backlight driver and current sinks.

This MFD patch supports the I2C communication using the regmap,
the interrupt handling using the linear IRQ domain and
configurable platform data structures for each driver module.

 (Driver Architecture)

                                         < mfd devices >
  LP8788 HW  ..........  mfd  .......... regulator drivers
                I2C                      power supply driver
                IRQs                     iio adc driver
                                         rtc driver
                                         backlight driver
                                         current sink drivers

  o regulators    : LDOs and BUCKs
  o power supply  : Battery charger
  o iio adc       : Battery voltage/temperature
  o rtc           : RTC and alarm
  o backlight
  o current sink  : LED and vibrator

All MFD device modules are registered by LP8788 MFD core driver.
For sharing ...