mfd: Move WM831x to generic IRQ

Replace the wm831x-local IRQ infrastructure with genirq, allowing access
to the diagnostic infrastructure of genirq and allowing us to implement
interrupt support for the GPIOs.  The switchover is done within the
wm831x specific IRQ API, further patches will convert the individual
drivers to use genirq directly.

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