dm: change dm_warn() message into debug() in uclass_add()

The command "dm uclass" tries to display all the UClasses, but
some of them might be disabled by Kconfig.

The function do_dm_dump_uclass() iterates over all the UClass IDs
and calls uclass_get() for each of them.  Then, it displays annoying
message "Cannot find uclass for id ..." every time it fails to get
the UClass.

As a result, we get much noisier log for the "dm uclass" command.

  => dm uclass
  uclass 0: root
  - * root_driver @ bfb54028, seq 0, (req -1)

  Cannot find uclass for id 1: please add the UCLASS_DRIVER() ...
  Cannot find uclass for id 2: please add the UCLASS_DRIVER() ...
  Cannot find uclass for id 3: please add the UCLASS_DRIVER() ...
  Cannot find uclass for id 4: please add the UCLASS_DRIVER() ...
  Cannot find uclass for id 5: please add the UCLASS_DRIVER() ...
  Cannot find uclass for id 6: please add the UCLASS_DRIVER() ...

This commit suppresses these warnings.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Simon Glass <sjg@chromium.org>