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: 77fcf3cf1251 ("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>
diff --git a/configs/uDPU_defconfig b/configs/uDPU_defconfig
index f2852ad..f7cd1a0 100644
--- a/configs/uDPU_defconfig
+++ b/configs/uDPU_defconfig
@@ -70,6 +70,7 @@
 CONFIG_SPI_FLASH_MTD=y
 CONFIG_PHYLIB_10G=y
 CONFIG_PHY_MARVELL=y
+CONFIG_PHY_FIXED=y
 CONFIG_PHY_GIGE=y
 CONFIG_E1000=y
 CONFIG_MVNETA=y