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]

SIMPLE_DEV_PM_OPS already defines constant dev_pm_ops, thus also fix
'duplicate const' sparse warning.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>