zynq: disable unused Vitesse PHY

zynq: disable unused Vitesse PHY

All devices supported by the Zynq target have either a Realtek or Marvell
PHY. The Vitesse PHY was enabled when the target was created (2d45ad07fc64).
It's not used here, so it's safe to disable it.

Ethernet PHYs used by individual devices are listed below.

Device			PHY
AVNET ZedBoard		Marvell 88E1518
Digilent Zybo		Realtek RTL8211E
Digilent Zybo Z7	Realtek	RTL8211E or RTL8211F
Xilinx ZC702		Marvell 88E1116R

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>
Link: https://github.com/openwrt/openwrt/pull/19969
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>