drm/omap: panel-dsi-cm: 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
   4669	    464	      0	   5133	   140d	panel-dsi-cm.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   4725	    400	      0	   5125	   1405	panel-dsi-cm.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>