ARM: dts: bcm2835: add index to the ethernet alias

An alias name should have an index number even when it is the only of its type.
This allows U-Boot to add the local-mac-address property. Otherwise U-Boot
skips the alias.

Fixes: 6a937927 ("ARM: bcm2835: dt: Add the ethernet to the device trees")
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Acked-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Eric Anholt <eric@anholt.net>