mfd: wm8994: Update to fully use irq_domain

Take advantage of the new regmap irq_domain support to dynamically
allocate interrupts, using regmap_irq_get_virq() rather than irq_base
to look up the interrupts. This means that most users should not need
to specify an irq_base at all.

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