i7core_edac: convert it to use struct device

Instead of relying on a complex logic inside the edac core to create
a "device tree-like" sysfs struct, just use device_add.

Reviewed-by: Aristeu Rozanski <arozansk@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>