ec_multisymbol: Add defines CDF_PROB_BITS, CDF_PROB_TOP

Change-Id: I6c1717ad82d05ebe22327aec6989af2c0db336e3
diff --git a/aom_dsp/prob.c b/aom_dsp/prob.c
index a98a4bc..7d51dbb 100644
--- a/aom_dsp/prob.c
+++ b/aom_dsp/prob.c
@@ -198,7 +198,7 @@
     nsymbs++;
   }
   /* Compute the probabilities of each symbol in Q15 */
-  tree_node_compute_probs(symb, 0, 32768);
+  tree_node_compute_probs(symb, 0, CDF_PROB_TOP);
   /* Extract the cdf, index, path and length */
   tree_node_extract(symb, 0, 0, cdf, index, path, len);
   /* Convert to CDF */