mfd: Add initial WM8958 support

The WM8958 is a derivative of the WM8994 which is register compatible
with the addition of some extra features, mostly in the CODEC side.
The major change visible at the MFD level is that rather than a single
DBVDD supply we now have three separate DBVDDs so we must request and
enable a different set of supplies.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>