mfd: ab8500-debugfs: Allow number of IRQs to be provided more dynamically

With the introduction of new AB* platforms, it's important to allow
as much code reuse as possible. By allowing a system's number of IRQs
to be dynamically passed, we can reuse almost all of the -debugfs
driver.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>