riscv: dts: microchip: 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.

Note on the Icicle Kit board, currently only emac1 is enabled so it
becomes the 'ethernet0'.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>