cosmetic: doc: driver-model: Do not number driver lists

Everytime a dead driver is removed from the list,
we must re-number. This is a painful task.

Try
  git show e53232250 -- doc/driver-model/UDM-serial.txt
  git show 6f62f4207 -- doc/driver-model/UDM-serial.txt
  git show b9f4bc34a -- doc/driver-model/UDM-serial.txt
to see what I mean.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/doc/driver-model/UDM-keyboard.txt b/doc/driver-model/UDM-keyboard.txt
index ef3761d..5babfc5 100644
--- a/doc/driver-model/UDM-keyboard.txt
+++ b/doc/driver-model/UDM-keyboard.txt
@@ -28,18 +28,18 @@
 III) Analysis of in-tree drivers
 --------------------------------
 
-  1) board/mpl/common/kbd.c
-  -------------------------
+  board/mpl/common/kbd.c
+  ----------------------
   This driver is a classic STDIO driver, no problem with conversion is expected.
   Only necessary change will be to move this driver to a proper location.
 
-  2) board/rbc823/kbd.c
-  ---------------------
+  board/rbc823/kbd.c
+  ------------------
   This driver is a classic STDIO driver, no problem with conversion is expected.
   Only necessary change will be to move this driver to a proper location.
 
-  3) drivers/input/keyboard.c
-  ---------------------------
+  drivers/input/keyboard.c
+  ------------------------
   This driver is special in many ways. Firstly because this is a universal stub
   driver for converting scancodes from i8042 and the likes. Secondly because the
   buffer is filled by various other ad-hoc implementations of keyboard input by