mfd: ab8500-debugfs: Export all AB8500 ADCs as debugfs nodes

Allow a user to take a glimpse into the inner workings of the
AB8500 Analogue-to-Digital Converters, via debugfs.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: John Beckett <john.beckett@stericsson.com>
Reviewed-by: Mattias WALLIN <mattias.wallin@stericsson.com>