dm: Add a No-op uclass

This uclass is intended for devices that do not need any features from the
uclass, including binding children.
This will typically be used by devices that are used to bind child devices
but do not use dm_scan_fdt_dev() to do it. That is for example the case of
several USB wrappers that have 2 child devices (1 for device and 1 for
host) but bind only one at a any given time.

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