net: phy: dp83867: allow RGMII_TXID/RGMII_RXID interface types

Currently dp83867 driver returns error if phy interface type
PHY_INTERFACE_MODE_RGMII_RXID is used to set the rx only internal
delay. Similarly issue happens for PHY_INTERFACE_MODE_RGMII_TXID.
Fix this by checking also the interface type if a particular delay
value is missing in the phy dt bindings. Also update the DT document
accordingly.

Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: David S. Miller <davem@davemloft.net>