mfd: Add platform data to support multiple WM831x devices per board

If a system contains multiple WM831x devices we need to pass a device
number through to the MFD so that we use unique device IDs when we
instantiate child devices. In order to get support for this into 2.6.39
add some platform data to support the configuration, but no implementation
as yet.

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