riscv: dts: sifive: 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.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>