drivers/edac: add edac_mc_find API

This simple patch adds an important CORE API for EDAC that EDAC drivers can
use to find their edac_mc control structure by passing a mem_ctl_info
'instance' value

Needed for subsequent patches

Signed-off-by: Douglas Thompson <dougthompson@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>