regulator: Add WM8400 regulator support

The WM8400 provides two programmable DCDC step-down (buck) convertors
and four low-dropout (LDO) regulators. This driver provides support for
runtime managment of these in the standard regulator API.

Support for configuration of the suspend and hibernate mode behaviour
of the regulators is not yet included.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>