edac: mpc85xx: fix MPC85xx dependency

Since commit 5753c082

 ("powerpc/85xx:
Kconfig cleanup"), there is no MPC85xx Kconfig symbol anymore, so the
driver became non-selectable.

This patch fixes the issue by switching to PPC_85xx symbol.

Signed-off-by: Anton Vorontsov <avorontsov@mvista.com>
Cc: Doug Thompson <dougthompson@xmission.com>
Cc: Peter Tyser <ptyser@xes-inc.com>
Cc: Dave Jiang <djiang@mvista.com>
Cc: Kumar Gala <galak@kernel.crashing.org>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>