kobject: convert s390 ipl.c to kobj_attr interface

This makes the code a bit simpler and and gets us one step closer to
deleting the deprecated subsys_attr code.

NOTE, this needs the next patch in the series in order to work properly.
This will build, but the sysfs files will not properly operate.

Thanks to Cornelia for the build fix on this patch.

Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Michael Holzheu <holzheu@de.ibm.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Volker Sameske <sameske@de.ibm.com>
Cc: Cornelia Huck <cornelia.huck@de.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>