net/macb: bindings doc/trivial: fix sama5d4 comment

On sama5d4, we only have a GEM IP that is configured to do 10/100 Mbits. So the
use of "Gigabit" can be confusing.

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