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>

Documentation/devicetree/bindings/net/macb.txt

@@ -8,7 +8,7 @@ Required properties:
   Use "cdns,pc302-gem" for Picochip picoXcell pc302 and later devices based on
   the Cadence GEM, or the generic form: "cdns,gem".
   Use "atmel,sama5d3-gem" for the Gigabit IP available on Atmel sama5d3 SoCs.
-  Use "atmel,sama5d4-gem" for the Gigabit IP available on Atmel sama5d4 SoCs.
+  Use "atmel,sama5d4-gem" for the GEM IP (10/100) available on Atmel sama5d4 SoCs.
   Use "cdns,zynqmp-gem" for Zynq Ultrascale+ MPSoC.
 - reg: Address and length of the register set for the device
 - interrupts: Should contain macb interrupt