mfd: Calibrate ab8500 gpadc using OTP values

The GPADC found in the AB8500 needs to be calibrated to work
properly. This is done by writing a number of special OTP
(one-time-programmable) registers at production. This patch
makes sure that these values are used to calibrate the returned
value from the GPADC so that it is correct.

Signed-off-by: Johan Palsson <johan.palsson@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>