mfd: ab8500-gpadc: Add CONFIG_PM_RUNTIME to runtime_suspend/runtime_resume

Add CONFIG_PM_RUNTIME to runtime_suspend/runtime_resume functions
to fix the build warnings when CONFIG_PM_RUNTIME is not selected.
This is because runtime PM callbacks defined by SET_RUNTIME_PM_OPS
are only used when the CONFIG_PM_RUNTIME is enabled.

drivers/mfd/ab8500-gpadc.c:870:12: warning: 'ab8500_gpadc_runtime_suspend' defined but not used [-Wunused-function]
drivers/mfd/ab8500-gpadc.c:878:12: warning: 'ab8500_gpadc_runtime_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>