devfreq: convert devfreq_class to use dev_groups

The dev_attrs field of struct class is going away soon, dev_groups
should be used instead.  This converts the devfreq_class code to use the
correct field.

Cc: MyungJoo Ham <myungjoo.ham@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>