kobject: convert pseries/power.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.

Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Manish Ahuja <mahuja@us.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>