ARM: stm32: Enable KSZ90x1 PHY driver on DHCOR

Enable KSZ9x01 PHY driver in DHCOR common configuration, since the
AV96 board has this PHY populated on the PCB.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Patrice Chotard <patrice.chotard@foss.st.com>
Cc: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>