arm: zynq: Add device-type property for zynq ethernet phy nodes

Mention device-type = "ethernet-phy", as qemu will need this in absence
of compatible.

Signed-off-by: Sai Pavan Boddu <saipava@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>