Documentation: remove 'mach' from dontdiff file

The mach entry in the dontdiff file causes all the
arch/arm/mach-*/include/mach directories to be skipped.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>