mfd: tps65912: Add driver for the TPS65912 PMIC

This patch adds support for TPS65912 PMIC MFD core. It provides
communication through the I2C and SPI interfaces. It contains
the following components:

 - Regulators
 - GPIO controller

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