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>