dm: core: Allow iterating devices without uclass_get()

At present we have uclass_foreach_dev() which requires that uclass_get()
be called beforehand to find the uclass. This is good if we suspect that
that function might fail, but often we know that the uclass is available.

Add a new helper which does this uclass_get() automatically, so that only
the uclass ID is needed.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>