regulator: new drivers for AD5398 and AD5821

The AD5398 and AD5821 are single 10-bit DAC with 120 mA output current
sink capability. They feature an internal reference and operates from
a single 2.7 V to 5.5 V supply.

This driver supports both the AD5398 and the AD5821.  It adapts into the
voltage and current framework.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>