macb: Update compatibility string for SiFive FU540-C000

Update the compatibility string for SiFive FU540-C000 as per the new
string updated in the binding doc.
Reference:
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>
Tested-by: Paul Walmsley <paul.walmsley@sifive.com>
Signed-off-by: David S. Miller <davem@davemloft.net>