mfd: tps65912: Add new mfd device

The tps65912 chip is a power management IC. It contains the following
components:

- Regulators
- GPIO controller

The core driver is registered as a platform driver, it provides communication
through I2C and SPI interfaces.

Signed-off-by: Margarita Olaya Cabrera <magi@slimlogic.co.uk>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>