mfd: ab8500-gpadc: Add CONFIG_PM_SLEEP to suspend/resume

Add CONFIG_PM_SLEEP to suspend/resume functions to fix the build
warnings when CONFIG_PM_SLEEP is not selected. This is because
sleep PM callbacks defined by SET_SYSTEM_SLEEP_PM_OPS are only used
when the CONFIG_PM_SLEEP is enabled.

drivers/mfd/ab8500-gpadc.c:891:12: warning: 'ab8500_gpadc_suspend' defined but not used [-Wunused-function]
drivers/mfd/ab8500-gpadc.c:903:12: warning: 'ab8500_gpadc_resume' defined but not used [-Wunused-function]

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>