CfT: Revised CPSW driver for BeagleBone

[Tim Kientzle]

I have another iteration of the BeagleBone network driver
almost ready to commit and would appreciate any
feedback.

The code is on github; you can browse it at

  https://github.com/kientzle/cpsw

or download a snapshot from:

  https://github.com/kientzle/cpsw/archive/master.zip

Just copy the three if_cpsw* files to sys/arm/ti/cpsw/
and rebuild.

Here are the improvements since my last update:
  * Finally (!!) fixed the regular watchdog timeouts
     (turned out to be misconfigured Ethernet flow control)
  * TX performance improvements:
         = Eliminated TX interrupt
         = queue fragmented packets directly without defragmenting
  * Debugging: Connected up the "Host Error" interrupt to print
     detailed traces whenever the controller gets confused.
  * Debugging:  "sysctl dev.cpsw" gives a variety of statistics,
     including packet error counters, queue and watchdog statistics

Unless something shows up, I expect to commit it to
FreeBSD-CURRENT this coming weekend.

Tim