hwtracing: coresight: 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
   2573	    288	    296	   3157	    c55	coresight-etm-perf.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   2613	    224	    296	   3133	    c3d	coresight-etm-perf.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>