net: ti: cpsw: add hibernation callbacks

Setting a dev_pm_ops suspend/resume pair but not a set of
hibernation functions means those pm functions will not be
called upon hibernation.
Fix this by using SIMPLE_DEV_PM_OPS, which appropriately
assigns the suspend and hibernation handlers and move
cpsw_suspend/resume calbacks under CONFIG_PM_SLEEP
to avoid build warnings.

Signed-off-by: Grygorii Strashko <Grygorii.Strashko@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>