net: phy: Define Marvell 88e1518 phy

This phy is used on zedboard (xilinx zynq platform).

Signed-off-by: Michal Simek <monstr@monstr.eu>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Tom Rini <trini@ti.com>