EDAC: Octeon: Add error injection support

This adds an ad-hoc error injection method. Octeon II doesn't have
hardware support for injection, so this simulates it.

Signed-off-by: Daniel Walker <dwalker@fifo99.com>
Cc: David Daney <david.daney@cavium.com>
Cc: Doug Thompson <dougthompson@xmission.com>
Cc: linux-edac@vger.kernel.org
Cc: linux-mips@linux-mips.org
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/5873/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>