amd-xgbe: Add support for MDIO attached PHYs

Use the phylib support in the kernel to communicate with and control an
MDIO attached PHY. Use the hardware's MDIO communication mechanism to
communicate with the PHY.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>