net: dsa: mv88e6xxx: Add eeprom-length to binding

The binding documentation for the mv88e6xxx switch is missing the
eeprom-length property, which has been implemented since May 2016,
commit f8cd8753 ("dsa: mv88e6xxx: Handle eeprom-length property")
Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/dsa/marvell.txt

@@ -26,6 +26,10 @@ Optional properties:
 - interrupt-controller	: Indicates the switch is itself an interrupt
 			  controller. This is used for the PHY interrupts.
 #interrupt-cells = <2>	: Controller uses two cells, number and flag
+- eeprom-length		: Set to the length of an EEPROM connected to the
+			  switch. Must be set if the switch can not detect
+			  the presence and/or size of a connected EEPROM,
+			  otherwise optional.
 - mdio			: Container of PHY and devices on the switches MDIO
 			  bus.
 - mdio?		: Container of PHYs and devices on the external MDIO