Add a device that attaches to the memory controller. If ECC is enabled in

Add a device that attaches to the memory controller.  If ECC is enabled in
the BIOS the device will poll the controller once a second and report
any ECC corrections or uncorrectable errors.

This module is not loaded by default.  To load the module, add the following
line to /boot/loader.conf.  The module can only be loaded at boot-time:

    ecc_load="YES"'

As of this commit, only the AMD64's memory controller is supported.