stmmac: intel: add pcs-xpcs for Intel mGbE controller

Intel mGbE controller such as those in EHL & TGL uses pcs-xpcs driver for
SGMII interface. To ensure mdio bus scanning does not assign phy_device
to MDIO-addressable entities like intel serdes and pcs-xpcs, we set up
to phy_mask to skip them.

Signed-off-by: Ong Boon Leong <boon.leong.ong@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>