mfd: Support dynamic allocation of IRQ range for wm831x

Use irq_allocate_desc() to get the IRQ range, which turns into a noop on
non-sparse systems. Since all existing users are non-sparse there should
be no compatibility issues.

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