mfd: ab8500-gpadc: Add runtime pm support

Add runtime pm support to speed up multiple ADC reads in a row.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Reviewed-by: Ulf HANSSON <ulf.hansson@stericsson.com>