MAINTAINERS: merge s3c6400 and 6410 to 64xx

Removed by commit 431107ea ("ARM: S3C64XX: Merge mach-s3c6400 and
mach-s3c6410").

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>