mfd: lp873x: Add lp873x PMIC support

mfd: lp873x: Add lp873x PMIC support

The LP873X chip is a power management IC for Portable Navigation Systems
    and Tablet Computing devices. It contains the following components:

     - Regulators.
     - Configurable General Purpose Output Signals (GPO).

PMIC interacts with the main processor through i2c. PMIC has
couple of LDOs (Linear Regulators), couple of BUCKs (Step-Down DC-DC
Converter Cores) and GPOs (General Purpose Output Signals).

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>