phylib: add RGMII-ID interface mode definition

The RGMII spec allows compliance for devices that implement an internal
delay on TXC or RXC inside the transmitter.  This patch adds an RGMII_ID
definition to support RGMII-ID devices in the phylib.

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>