macb: bindings doc: update sifive fu540-c000 binding

As per the discussion with Nicolas Ferre[0], rename the compatible property
to a more appropriate and specific string.

[0] https://lore.kernel.org/netdev/CAJ2_jOFEVZQat0Yprg4hem4jRrqkB72FKSeQj4p8P5KA-+rgww@mail.gmail.com/

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Reviewed-by: Paul Walmsley <paul.walmsley@sifive.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>