DT: net: document Ethernet bindings in one place

This patch is an attempt to gather the Ethernet related bindings in one file,
like it's done in the MMC and some other subsystems. It should save some of
the trouble of documenting several properties over and over in each binding
document, instead only making reference to the main file.

I have used the Embedded Power Architecture(TM) Platform Requirements (ePAPR)
standard as a base for the properties description, also documenting some ad-hoc
properties that have been introduced over time despite having direct analogs in
ePAPR.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>