powerpc/opal: use default_groups in kobj_type

There are currently 2 ways to create a set of sysfs files for a
kobj_type, through the default_attrs field, and the default_groups
field.  Move the powerpc opal dump and elog sysfs code to use
default_groups field which has been the preferred way since aa30f47c
("kobject: Add support for default attribute groups to kobj_type") so
that we can soon get rid of the obsolete default_attrs field.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220104161318.1306023-1-gregkh@linuxfoundation.org