mvebu: uDPU: include fixed-phy support

uDPU relies on using fixed-phy for the SFP support, and since the
fixed-phy parsing was moved to the generic driver instead of mvneta
networking stopped working on uDPU with:
uDPU>> dhcp
dm_eth_phy_connect failed

This is due to the conversion commit not enabling fixed-phy support
in defconfig like it did for other boards.

Fixes: 77fcf3cf ("net: mvneta: Convert to use PHY_FIXED for fixed-link")
Signed-off-by: Robert Marko <robert.marko@sartura.hr>
Reviewed-by: Stefan Roese <sr@denx.de>