[POWERPC] EDAC ECC software scrubber

Implements the per arch atomic_scrub() that EDAC uses for software
ECC scrubbing.  It reads memory and then writes back the original
value, allowing the hardware to detect and correct memory errors.

Signed-off-by: Dave Jiang <djiang@mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>