regulator: Add WM8994 regulator support

The WM8994 contains two LDOs with mixed hardware/software control to
minimise the number of external supplies required while delivering
optimal voltages to minimise power consumption.

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