MFD: ucb1x00-core: 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/ucb1x00-core.c:672:12: warning: 'ucb1x00_suspend' defined but not used [-Wunused-function]
drivers/mfd/ucb1x00-core.c:704:12: warning: 'ucb1x00_resume' defined but not used [-Wunused-function]

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