net: macb: add sam9x60-macb compatibility string

Add a new compatibility string for this product. It's using
at91sam9260-macb layout but has a newer hardware revision: it's safer
to use its own string.

Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>