dm: core: Allow manual sequence numbering

dm: core: Allow manual sequence numbering

Some buses have their own rules which require assigning sequence numbers
with a bus-specific algorithm. For example, PCI requires that sub-buses
are numbered higher than their parent buses, meaning effectively that
parent buses must be numbered only after all of their child buses have
been numbered.

Add a uclass flag to indicate that driver model should not assign sequence
numbers. In this case, the uclass must do it.

Signed-off-by: Simon Glass <sjg@chromium.org>