Add /dev/oldmem to devices.txt documentation

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

Documentation/devices.txt

@@ -94,6 +94,8 @@ Your cooperation is appreciated.
 		  9 = /dev/urandom	Faster, less secure random number gen.
 		 10 = /dev/aio		Asynchronous I/O notification interface
 		 11 = /dev/kmsg		Writes to this come out as printk's
+		 12 = /dev/oldmem	Used by crashdump kernels to access
+					the memory of the kernel that crashed.
 
   1 block	RAM disk
 		  0 = /dev/ram0		First RAM disk