Documentation: net: phy: Add links to several standards documents

Add links to the IEEE 802.3-2008 document, and the RGMII v1.3 and v2.0
revisions of the standard.

Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>