net: stmmac: add new switch to struct plat_stmmacenet_data

On some platforms, the PCS can be integrated in the MAC so the driver
will not see any PCS link activity. Add a switch that allows the platform
drivers to let the core code know.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Reviewed-by: Jose Abreu <Jose.Abreu@synopsys.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>