dt-bindings: phy: dp83867: Describe how driver behaves w.r.t rgmii delay

Add a note to make it more clear how the driver behaves when "rgmii" vs
"rgmii-id", "rgmii-idrx", or "rgmii-idtx" interface modes are selected.

Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/devicetree/bindings/net/ti,dp83867.txt

@@ -11,6 +11,14 @@ Required properties:
 	- ti,fifo-depth - Transmitt FIFO depth- see dt-bindings/net/ti-dp83867.h
 		for applicable values
 
+Note: If the interface type is PHY_INTERFACE_MODE_RGMII the TX/RX clock delays
+      will be left at their default values, as set by the PHY's pin strapping.
+      The default strapping will use a delay of 2.00 ns.  Thus
+      PHY_INTERFACE_MODE_RGMII, by default, does not behave as RGMII with no
+      internal delay, but as PHY_INTERFACE_MODE_RGMII_ID.  The device tree
+      should use "rgmii-id" if internal delays are desired as this may be
+      changed in future to cause "rgmii" mode to disable delays.
+
 Optional property:
 	- ti,min-output-impedance - MAC Interface Impedance control to set
 				    the programmable output impedance to