xilinx: Add CONFIG_DM_ETH_PHY config

Enable CONFIG_DM_ETH_PHY to utilize shared MDIO bus support on all xilinx
platforms.

Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Acked-by: Ashok Reddy Soma <ashok.reddy.soma@xilinx.com>
Link: https://lore.kernel.org/r/965981eb324d13a98aad8bd88eb8b50bc5147a7e.1652181968.git.michal.simek@amd.com