mfd: Force I2C to be built in when building WM831x

This works around issues with allmodconfig where it won't propagate the
dependency from the WM831x core back to the I2C and MFD cores.  When
doing allmodconfig this causes WM831x to be omitted and ensures that in
normal builds the dependencies get shaken out.

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