ASoC: WM8903: Create default platform data structure

When no platform data is supplied, point pdata at a default platform
structure. This enables two future changes:

a) Defines the default platform data values in a single place.
b) There is always a valid pdata pointer, so some conditional code can
   be simplified by a later patch.

Based on work by John Bonesio, but significantly reworked since then.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>