mfd: ab8500-gpadc: Implemented suspend/resume

suspend/resume methods implemented to prevent suspend while the gpadc
driver is busy.

Signed-off-by: Daniel WILLERUD <daniel.willerud@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Jonas ABERG <jonas.aberg@stericsson.com>
Reviewed-by: Ulf HANSSON <ulf.hansson@stericsson.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>