dm: print the index of the device when dumping the dm tree

Command "dm tree" dumps the devices with class, driver, name information.
Add the index of the device in the class too, because the information is
useful for the bind/unbind commands.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>