Import ioat(4) driver

I/OAT is also referred to as Crystal Beach DMA and is a Platform Storage
Extension (PSE) on some Intel server platforms.

This driver currently supports DMA descriptors only and is part of a
larger effort to upstream an interconnect between multiple systems using
the Non-Transparent Bridge (NTB) PSE.

For now, this driver is only built on AMD64 platforms. It may be ported
to work on i386 later, if that is desired. The hardware is exclusive to
x86.

Further documentation on ioat(4), including API documentation and usage,
can be found in the new manual page.

Bring in a test tool, ioatcontrol(8), in tools/tools/ioat. The test
tool is not hooked up to the build and is not intended for end users.

Submitted by: jimharris, Carl Delsey <carl.r.delsey@intel.com>
Reviewed by: jimharris (reviewed my changes)
Approved by: markj (mentor)
Relnotes: yes
Sponsored by: Intel
Sponsored by: EMC / Isilon Storage Division
Differential Revision: https://reviews.freebsd.org/D3456