dts: Add a binding for Synopsys emac max-frame-size

This change adds a parameter for the Synopsys 10/100/1000
stmmac Ethernet driver to configure the maximum frame
size supported by the EMAC driver. Synopsys allows the FIFO
sizes to be configured when the cores are built for a particular
device, but do not provide a way for the driver to read
information from the device about the maximum MTU size
supported as limited by the device's FIFO size.

Signed-off-by: Vince Bridgers <vbridgers2013@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/stmmac.txt

@@ -32,6 +32,8 @@ Optional properties:
 - resets: Should contain a phandle to the STMMAC reset signal, if any
 - reset-names: Should contain the reset signal name "stmmaceth", if a
 	reset phandle is given
+- max-frame-size:	Maximum Transfer Unit (IEEE defined MTU), rather
+			than the maximum frame size.
 
 Examples:
 
@@ -42,5 +44,6 @@ Examples:
 		interrupts = <24 23>;
 		interrupt-names = "macirq", "eth_wake_irq";
 		mac-address = [000000000000]; /* Filled in by U-Boot */
+		max-frame-size = <3800>;
 		phy-mode = "gmii";
 	};