regulator: tps65218: Add Regulator driver for TPS65218 PMIC

This patch adds support for TPS65218 PMIC regulators.

The regulators set consists of 6 DCDCs and 1 LDO. The output
voltages are configurable and are meant to supply power to the
main processor and other components.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>