mfd: Refactor ab8500 GPADC API, add raw access

Refactor the GPADC interface to avoid bugs in calling code:

- ab8500_gpadc_[convert|read_raw|ad_to_voltage] clarifies
  each functions use case, *convert wraps *read_raw, and we
  can access raw ADC values properly.
- Renamed gpadc function arguments from "input" to "channel" to
  clarify use, so we don't get confused again.

Signed-off-by: Kalle Komierowski <kalle.komierowski@stericsson.com>
Reviewed-by: Mattias Wallin <mattias.wallin@stericsson.com>
Reviewed-by: John Beckett <john.beckett@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>