dm: spi: Move slave details to child platdata

At present we go through various contortions to store the SPI slave's chip
select in its private data. This only exists when the slave is active so
must be set up when it is probed. Until the device is probed we don't
actually know what chip select it will appear on.

However, now that we can support per-child platform data, we can use that
instead. This allows us to set up the chip select when the child is bound,
and avoid the messy contortions.

Unfortunately this is a fairly large change and it seems to be difficult to
break it down further.

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