drm: armada: constify drm_prop_enum_list structures.

drm_prop_enum_lists are not supposed to change at runtime. All functions
working with drm_prop_enum_list provided by <drm/drm_property.h> work
with
const drm_prop_enum_list. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   9629	    744	      0	  10373	   2885 drivers/gpu/drm/armada/armada_crtc.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   9757	    616	      0	  10373	   2885 drivers/gpu/drm/armada/armada_crtc.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/d2344c4dc40238cfe48fa6d917767df0f053150a.1498902844.git.arvind.yadav.cs@gmail.com