dm: spi: Set up the spi_slave device pointer in child_pre_probe()

At present we use struct spi_slave as our device pointer in a lot of places
to avoid changing the old SPI API. At some point this will go away.

But for now, it is better if the SPI uclass sets up this pointer, rather
than relying on passing it into the device when it is probed. We can use the
new uclass child_pre_probe() method to do this.

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