dm: core: Show device sequence instead in dm_dump_tree()

Currently uclass index is shown in DM tree dump which ignores alias
sequence numbering. The result could be confusing since these 2 numbers
could be different. Show device sequence number instead as it's more
meaningful.

Also update documentation to use sequence number.

Signed-off-by: Zixun LI <admin@hifiphile.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/usage/cmd/dm.rst

@@ -112,9 +112,8 @@ This shows the full tree of devices including the following fields:
 uclass
     Shows the name of the uclass for the device
 
-Index
-    Shows the index number of the device, within the uclass. This shows the
-    ordering within the uclass, but not the sequence number.
+Seq
+    Shows the sequence number of the device, within the uclass.
 
 Probed
     Shows `+` if the device is active
@@ -366,7 +365,7 @@ dm tree
 This example shows the abridged sandbox output::
 
     => dm tree
-    Class     Index  Probed  Driver                Name
+    Class     Seq    Probed  Driver                Name
     -----------------------------------------------------------
     root          0  [ + ]   root_driver           root_driver
     demo          0  [   ]   demo_shape_drv        |-- demo_shape_drv