mfd: Register WM8400 codec device

Register a child device for the codec in the WM8400.

Also switch the unregistration of the MFD devices to use the MFD core
since the current code is hand rolling the same thing.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@openedhand.com>