dt-bindings: net: lan966x: Accept standard ethernet prefixes

The dsa.yaml and ethernet-switch.yaml bindings recommend
prefixing ethernet switches and ports with "ethernet-" so
make the LAN966x do the same.

Reported-by: Herve Codina <herve.codina@bootlin.com>
Signed-off-by: Linus Walleij <linusw@kernel.org>