wm831x_power: Use genirq

Since the WM831x core has been converted to use genirq for the
interrupt controller there is no longer any need for chip specific
wrappers for IRQ operations. Convert to use genirq directly.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Anton Vorontsov <cbouatmailru@gmail.com>