mfd: Fix wm831x register range passing for recent ARM updates

mfd: Fix wm831x register range passing for recent ARM updates

The removal of mach/io.h from most ARM platforms also set the range of
valid IO ports to be empty for most platforms when previously any 32
bit integer had been valid. This makes it impossible to add IO resources
as the added range is smaller than that of the root resource for IO ports.

Since we're not really using IO memory at all fix this by defining our
own root resource outside the normal tree and make that the parent of
all IO resources. This also ensures we won't conflict with read IO ports
if we ever run on a platform which happens to use them.

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