ACPI / DPTF: constify attribute_group structures

attribute_groups are not supposed to change at runtime. All functions
working with attribute_groups provided by <linux/sysfs.h> work with const
attribute_group. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
    904	    496	      0	   1400	    578	drivers/acpi/dptf/dptf_power.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
    944	    432	      0	   1376	    560	drivers/acpi/dptf/dptf_power.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>