riscv: dts: fix network

This MR fixes issue #1 (closed) and all the network related commands, such as dhcp, tftpboot et. al.

Credit goes to @rlkeck, who put the solution together.

I marked it as draft because I'm not completely happy with it and open to discussion.

1. Naming: Shouldn't we change the name to beaglev-fire.dts instead? I find it a bit bad to squash an existing board and won't it lead to merge issues upstream?

2. I don't quite understand what's going on with this line. Why do we need to reference 3 different dts's? Shouldn't one unified dts be enough?