tps65912: add regulator driver

The tps65912 consist of 4 DCDCs and 10 LDOs. The output voltages can be
configured by the SPI or I2C interface, they are meant to supply power
to the main processor and other components.

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