gpiolib: wm8994: Use irq_domain mappings for gpios

This has no practical impact at present since we don't support device tree
so any user must have set an irq_base but this will in future allow a
transition to device tree with minimal invasiveness.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>