mfd: Add WM831x interrupt support

The WM831x includes an interrupt controller managing interrupts for
the various functions on the chip. This patch adds support for the
core interrupt block on the device.

Ideally this would be supported by genirq, particularly for the
GPIOs, but currently genirq is unable to cope with controllers on
interrupt driven buses so we cut'n'paste the generic interface.
Once genirq is able to cope chips like this it should be a case
of filing the prefixes off the code and redoing wm831x-irq.c to
move over.

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