ACPI / dock: constify attribute_group structure

Functions working with attribute_groups provided by <linux/sysfs.h>
work with const attribute_group. These attribute_group structures do not
change at runtime so mark them as const.

File size before:
 text      data     bss     dec     hex filename
 7902      3528      64   11494    2ce6 drivers/acpi/dock.o

File size after:
 text      data     bss     dec     hex filename
 7998      3432      64   11494    2ce6 drivers/acpi/dock.o

This change was made with the help of Coccinelle.

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>