[ARM] 3420/1: Missing clobber in example code

Patch from Paul Brook

The example code in the source documentation for __kernel_dmb
clobbers r0 but doesn't list it the asm clobber list.

Signed-off-by: Paul Brook <paul@codesourcery.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>