s390/prng: let misc_register() add the prng sysfs attributes

Instead of creating the sysfs attributes for the prng devices by hand,
describe them in .groups and let the misdevice core handle it.

This also ensures that the attributes are available when the KOBJ_ADD
event is raised.

Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Reviewed-by: Harald Freudenberger <freude@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>