[9fans] 9atom hw support

[erik quanstrom]

i've taken a first stab at documenting some
9atom hardware differences and hardware
known to be in production use here
        http://www.quanstro.net/plan9/9atom
please yell at me offline where i have missed
stuff.  for the ide driver, the support often comes
in the form of an added vid/did or a correction
to the vid/did list

i hope to document some of the non-hw differences
soon.  you can preemtively yell at me about this, too—
offline

- erik