net: zynq_gem: Use shared MDIO bus support for zynqmp

CONFIG_ETH_PHY enables support to utilize generic ethernet phy
framework. Though if ethernet PHY node is in other ethernet node, it
will use shared MDIO to access the PHY of other ethernet.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: T Karthik Reddy <t.karthik.reddy@xilinx.com>
Link: https://lore.kernel.org/r/337b1a38ba36cde1951739af62fb3d2736d97f53.1648631275.git.michal.simek@xilinx.com