mfd: tps65086: Add driver for the TPS65086 PMIC

Add support for the TPS65912 device. It provides communication
through I2C and contains the following components:

 - Regulators
 - Load switches
 - GPO controller

Signed-off-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>