riscv: sifive_u: Add ethernet0 to the aliases node

riscv: sifive_u: Add ethernet0 to the aliases node

U-Boot expects this alias to be in place in order to fix up the mac
address of the ethernet node.

This is to keep in sync with Linux kernel commit below:
https://patchwork.kernel.org/patch/11133033/

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>