regulator: da9210: New driver

I2C driver for the Dialog DA9210 Multi-phase 12A DC-DC Buck.

Signed-off-by: Steve Twiss <stwiss.opensource@diasemi.com>
Signed-off-by: David Dajun Chen <david.chen@diasemi.com>
Signed-off-by: Mark Brown <broonie@linaro.org>