net/macb: Update DT bindings documentation

Add missing "cdns,at91sam9260-macb", "atmel,sama5d3-gem" and
"atmel,sama5d4-gem" compatible strings.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reviewed-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>