net: stmmac: split xPCS setup from mdio register

This patch is a preparation patch for the enabling of Intel mGbE 2.5Gbps
link speed. The Intel mGbR link speed configuration (1G/2.5G) is depends on
a mdio ADHOC register which can be configured in the bios menu.
As PHY interface might be different for 1G and 2.5G, the mdio bus need be
ready to check the link speed and select the PHY interface before probing
the xPCS.

Signed-off-by: Voon Weifeng <weifeng.voon@intel.com>
Signed-off-by: Michael Sit Wei Hong <michael.wei.hong.sit@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>