mfd: ab8500-gpadc: Add gpadc hw conversion

Add the support of gpacd hw conversion and make the number of
sample configurable.

Signed-off-by: M'boumba Cedric Madianga <cedric.madianga@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Mattias WALLIN <mattias.wallin@stericsson.com>
Tested-by: Michel JAOUEN <michel.jaouen@stericsson.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>