Driver Core: add class iteration api

Add the following class iteration functions for driver use:
	class_for_each_device
	class_find_device
	class_for_each_child
	class_find_child

Signed-off-by: Dave Young <hidave.darkstar@gmail.com>
Acked-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>