EDAC/mpc85xx: Prevent building as a module

The mpc85xx EDAC driver can be configured as a module but then fails to
build because it uses two unexported symbols:

  ERROR: ".pci_find_hose_for_OF_device" [drivers/edac/mpc85xx_edac_mod.ko] undefined!
  ERROR: ".early_find_capability" [drivers/edac/mpc85xx_edac_mod.ko] undefined!

We don't want to export those symbols just for this driver, so make the
driver only configurable as a built-in.

This seems to have been broken since at least

  c92132f5 ("edac/85xx: Add PCIe error interrupt edac support")

(Nov 2013).

 [ bp: make it depend on EDAC=y so that the EDAC core doesn't get built
   as a module. ]

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Johannes Thumshirn <jth@kernel.org>
Cc: James Morse <james.morse@arm.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: linux-edac <linux-edac@vger.kernel.org>
Cc: linuxppc-dev@ozlabs.org
Cc: morbidrsa@gmail.com
Link: https://lkml.kernel.org/r/20190502141941.12927-1-mpe@ellerman.id.au