PATCH: acpi ec burst mode

Please try out the patch at:
   http://root.org/~nate/freebsd/ec-burst.diff

This adds support for the embedded controller to use burst mode by
default.  It removes our non-working EventWaitIntr() implementation.  It
has a debug print that will show:
   "EC Waited max 2 cycles"

Please report the maximum value you get after applying the patch and
running it for a while.  Also please report any regression in
functionality.  You can turn off burst mode (on by default with this
patch) with:
   sysctl hw.acpi.burst_mode=0

You should run this with "hw.acpi.verbose=1" in /boot/loader.conf.  If you
get any Embedded Controller errors, please let me know.

Thanks,
-Nate