Add /dev/oldmem to devices.txt documentation

Signed-off-by: Dave Jones <davej@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: <device@lanana.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>