regulator: tps65912: Add regulator driver for the TPS65912 PMIC

This patch adds support for TPS65912 PMIC regulators.

The regulators set consists of 4 DCDCs and 10 LDOs. The output
voltages are configurable and are meant to supply power to the
main processor and other components.

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