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: NMartin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: NFlorian Fainelli <f.fainelli@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/phy.txt

@@ -407,3 +407,13 @@ Board Fixups
  The stubs set one of the two matching criteria, and set the other one to
  match anything.
 
+Standards
+
+ IEEE Standard 802.3: CSMA/CD Access Method and Physical Layer Specifications, Section Two:
+ http://standards.ieee.org/getieee802/download/802.3-2008_section2.pdf
+
+ RGMII v1.3:
+ http://web.archive.org/web/20160303212629/http://www.hp.com/rnd/pdfs/RGMIIv1_3.pdf
+
+ RGMII v2.0:
+ http://web.archive.org/web/20160303171328/http://www.hp.com/rnd/pdfs/RGMIIv2_0_final_hp.pdf