mfd: ab8500-debugfs: Add interrupt debug

This patch adds an entry in debugfs to check number of interrupts
from the AB.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Bengt Jonsson <bengt.g.jonsson@stericsson.com>
Reviewed-by: Rabin VINCENT <rabin.vincent@stericsson.com>