mfd: ab8500: Debugfs code depends on gpadc

The AB8500_DEBUG code specifically requires access to the gpadc code,
not just the common ab8500 driver.

drivers/built-in.o: In function `ab8500_gpadc_bat_ctrl_print':
 mfd/ab8500-debugfs.c:1733: undefined reference to `ab8500_gpadc_get'
 mfd/ab8500-debugfs.c:1734: undefined reference to `ab8500_gpadc_read_raw'
 mfd/ab8500-debugfs.c:1736: undefined reference to `ab8500_gpadc_ad_to_voltage'

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>