net: phy: xilinx: Drop #ifdef CONFIG_DM_ETH around phy_connect_gmii2rgmii()

net: phy: xilinx: Drop #ifdef CONFIG_DM_ETH around phy_connect_gmii2rgmii()

At present phy_connect_gmii2rgmii() is implemented using a DM API
dev_of_offset() hence it cannot support a non-DM configuration.
Remove the non-DM version prototype of phy_connect_gmii2rgmii()
and make the driver depend on CONFIG_DM_ETH.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>