Driver Core: Warn driver authors about adding device attributes

Add a blurb to the driver-model documentation about how (not) to add
extra attributes to a struct device at driver probe time.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>