ARM: 7201/1: add EDAC atomic_scrub function

Add support for architecture specific EDAC atomic_scrub to ARM. Only ARMv6+
is implemented as ldrex/strex instructions are needed. Supporting EDAC on
ARMv5 or earlier is unlikely at this point anyway.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>