Some device types can have child devices and want to store information
about them. For example a USB flash stick attached to a USB host
controller would likely use this space. The controller can hold
information about the USB state of each of its children.

The data is stored attached to the child device in the 'parent_priv'
member. It can be auto-allocated by dm when the child is probed. To
do this, add a per_child_auto_alloc_size value to the parent driver.

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