mfd: Move ab8500 gpadc header to subdir

This moves the ab8500-gpadc.h header down into the ab8500/
subdir in include/linux/mfd and fixes some whitespace in the
header in the process.

Signed-off-by: Daniel Willerud <daniel.willerud@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>