dm: i2c: Move slave details to child platdata

At present we go through various contortions to store the I2C's chip
address in its private data. This only exists when the chip is active so
must be set up when it is probed. Until the device is probed we don't
actually record what address 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 address 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>
Reviewed-by: Masahiro Yamada <yamada.m@jp.panasonic.com>