omnia needs MARVELL_PHY for the WAN port to work

interestingly, we only see this when it boots from mmc, because
for tftpboot the bootloader has already performed negotiation
and therefore it works despite the missing option.

devices/turris-omnia/default.nix

@@ -130,6 +130,7 @@
             MVNETA_BM_ENABLE = "y";
             SRAM = "y"; # mmio-sram is "compatible" for bm_bppi reqd by BM
             PHY_MVEBU_A38X_COMPHY = "y"; # for eth2
+            MARVELL_PHY = "y";
 
             MVPP2 = "y";
             MV_XOR = "y";