include/asm-:mips add missing edac h file

EDAC has a foundation to perform software memory scrubbing, but it requires a
per architecture (atomic_scrub) function for performing an atomic update
operation.  Under X86, this is done with a

lock:  add  [addr],0

in the file asm-x86/edac.h

This patch provides the MIPS arch with that atomic function, atomic_scrub() in

asm-mips/edac.h

Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: Doug Thompson <dougthompson@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>