mfd: sec-core: sec_pmic_{suspend,resume}() should depend on CONFIG_PM_SLEEP

If CONFIG_PM_SLEEP=n:

drivers/mfd/sec-core.c:349: warning: ‘sec_pmic_suspend’ defined but not used
drivers/mfd/sec-core.c:371: warning: ‘sec_pmic_resume’ defined but not used

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>