MAINTAINERS: update file pattern for ARC uart

Commit 6659a20a

 ("ARC: MAINTAINERS update for ARC") typoed the file
pattern.  Fix it.

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