regulator: Add support for DA9121 regulator

Add support for the Dialog Semiconductor DA9121, a single-channel
dual-phase buck converter controlled via I2C.

Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Link: https://lore.kernel.org/r/20201103100021.19603-3-vincent.whitchurch@axis.com
Signed-off-by: Mark Brown <broonie@kernel.org>