mfd: Guard tc3589x PM methods with CONFIG_PM_SLEEP
Guard PM methods with CONFIG_PM_SLEEP and get rid of some unneeded #ifdefs.
This fixes below build warnings:
CC drivers/mfd/tc3589x.o
drivers/mfd/tc3589x.c:361:12: warning: 'tc3589x_suspend' defined but not used [-Wunused-function]
drivers/mfd/tc3589x.c:375:12: warning: 'tc3589x_resume' defined but not used [-Wunused-function]