dm: device: Request next sequence number

For CONFIG_OF_PRIOR_STAGE, in the absence of a device tree alias for a
given device, use the next request number for that type of device.
This allows aliases to be used when they're available, while still
allowing unaliased devices to be probed.

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