bus: arm-ccn: 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
   9074	   5592	    416	  15082	   3aea	drivers/bus/arm-ccn.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   9327	   5336	    416	  15079	   3ae7	drivers/bus/arm-ccn.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Pawel Moll <pawel.moll@arm.com>