thermal: int340x: 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
   1687	    592	      0	   2279	    8e7	int3400_thermal.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   1751	    528	      0	   2279	    8e7	int3400_thermal.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>