EDAC, fsl: Move error injection under CONFIG_EDAC_DEBUG

Gate error injection feature with CONFIG_EDAC_DEBUG so that it is not
visible in production setups.

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: York Sun <york.sun@nxp.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: linux-edac <linux-edac@vger.kernel.org>
Link: https://lkml.kernel.org/r/20181119225303.13265-1-york.sun@nxp.com