net: stmmac: allow to split suspend/resume from init/exit callbacks

Let the stmmac platform drivers provide dedicated suspend and resume
callbacks rather than always re-using the init and exits callbacks.
If the driver does not provide the suspend or resume callback, we fall
back to the old behavior trying to use exit or init.

This allows a specific platform to perform only a partial power-down on
suspend if Wake-on-Lan is enabled but always perform the full shutdown
sequence if the module is unloaded.

Signed-off-by: Vincent Palatin <vpalatin@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>