atmel_mxt - Add atmel MXT touchscreen driver (Acer c720p)

* Implements the communications infrastructure on the hardware backend and
  emulates an elographics tablet on the frontend.

* X Configuration is:

    Section "InputDevice"
	    Identifier  "TouchScreen0"
	    Driver      "elographics"
	    Option      "Device" "/dev/atmel1-4a"
    #       Option      "ZAxisMapping" "4 5 6 7"
    EndSection

* System configuration is (in /boot/loader.conf):

    ig4_load="YES"
    atmel_mxt_load="YES"

* Currently only tested and only likely to work with the Acer c720p.
  Can do basic touchscreen operations... hold-move-and-release for
  button-1.  Can move or resize windows (you may want to make your resize
  widgets bigger).  Can hit buttons and hotlinks in browsers.  Cannot
  scroll a browser window on its own.

* Noise issues.  I've noticed that both my chromebooks have a lot of mouse
  pointer jitter with both the trackpad and the touchscreen, more than
  I can actually fix.  I've noticed some complaints on the linux forums too
  so it aint just us.

  The jitter is reduced if you put the laptop on your lap, and appears to
  go away entirely if the laptop is on your lap AND the power is disconnected
  (i.e. running on battery).
drm/i915: Only build on amd64/x86_64 machines

This driver now requires a direct memory map.