This updates some driver data documentation:

 - removes references to some fields that haven't been there for a
   long time now, e.g. pre-kobject or even older;

 - giving more information about the probe() method;

 - adding an example of how platform_data is used
Signed-off-by: NDavid Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: NGreg Kroah-Hartman <gregkh@suse.de>