mpc85xx_edac: convert sysfs logic to use struct device

Now that the EDAC core supports struct device, there's no sense on
having any logic at the EDAC core to simulate it. So, instead of adding
such logic there, change the logic at mpc85xx_edac to use it

compile-tested only.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>