mfd: Allow touchscreen to be disabled on wm831x devices

Allow platform data to flag the touchscreen as disabled so that if the
touch driver is built in we don't end up causing lots of work by spuriously
detecting touchscreen activity on systems where it isn't in use.

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