PM / QoS: constify *_attribute_group.

File size before:
   text	   data	    bss	    dec	    hex	filename
   3890	   1152	      8	   5050	   13ba	drivers/base/power/sysfs.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   4250	    800	      8	   5058	   13c2	drivers/base/power/sysfs.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>