ASoC: Refactor WM8731 device registration

Move the WM8731 driver to use a more standard device registration
scheme where the device can be registered independantly of the ASoC
probe.

As a transition measure push the current manual code for registering
the WM8731 into the individual machine driver probes. This allows
separate patches to update the relevant architecture files with less
risk of merge issues.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>