I already found the problem. The comment in devicetree.cb was wrong. Device 1c.5 was not the NIC, but instead it's 1c.3.
I'm now using this:
device pci 1c.3 on # PCIe Port #4, Realtek PCIe GbE Controller device pci 00.0 on subsystemid 0x1849 0x1e16 end end
And that works fine:
eno0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP mode DEFAULT group default qlen 1000 altname enp3s0
I've uploaded the patch to gerrit: https://review.coreboot.org/c/coreboot/+/73516