arm64: dts: renesas: spider-ethernet: Add ethernetN aliases for rswitch

arm64: dts: renesas: spider-ethernet: Add ethernetN aliases for rswitch

The rswitch has three independent ports which each can act as a separate
interface with its own MAC address.  Describe DT aliases ethernet0,
ethernet1, ethernet2 for each rswitch port in DT.  This allows U-Boot to
insert MAC addresses from its environment variables
ethaddr/eth1addr/eth2addr into each rswitch port nodes, so Linux can
read and use one unique MAC address for each rswitch port.

Note that it is unlikely this would break existing rswitch driver
operation in the Linux kernel, because as of right now, the rswitch
driver already calls of_get_ethdev_address() for each port to read
out the MAC address of each rswitch port DT node.  If that is missing,
it falls back to MAC address settings read from the hardware itself.
If that also fails, it uses a random MAC address.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/20250118111344.361617-4-marek.vasut+renesas@mailbox.org
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>