dt-bindings: net: Add tx and rx internal delays

tx-internal-delays and rx-internal-delays are a common setting for RGMII
capable devices.

These properties are used when the phy-mode or phy-controller is set to
rgmii-id, rgmii-rxid or rgmii-txid.  These modes indicate to the
controller that the PHY will add the internal delay for the connection.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>