mfd: intel_soc_pmic: Add CONFIG_PM_SLEEP check for suspend_fn/resume_fn

This patch fix warning message with CONFIG_PM_SLEEP disabled
If CONFIG_PM_SLEEP is not enabled we receive the following warning message:

drivers/mfd/intel_soc_pmic_core.c:118:12:
 warning: 'intel_soc_pmic_suspend' defined but not used

Signed-off-by: Jaewon Kim <jaewon02.kim@samsung.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>