omapfb: panel-sony-acx565akm: constify attribute_group structures.

attribute_group are not supposed to change at runtime. All functions
working with attribute_group 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
   5227	    560	      0	   5787	   169b	panel-sony-acx565akm

File size after adding 'const':
   text	   data	    bss	    dec	    hex	filename
   5291	    496	      0	   5787	   169b	panel-sony-acx565akm.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>