net: stmmac: Power up SERDES after the PHY link

The Tegra MGBE ethernet controller requires that the SERDES link is
powered-up after the PHY link is up, otherwise the link fails to
become ready following a resume from suspend. Add a variable to indicate
that the SERDES link must be powered-up after the PHY link.

Signed-off-by: Revanth Kumar Uppala <ruppala@nvidia.com>
Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>