Added pcbb_xoff parameters for dualtor-aa-56 topo and 50000_40m profile (sonic-net#13564)

Description of PR
Summary: Add pcbb_xoff parameters for dualtor-aa-56 topo and 50000_40m profile for Arista-7260CX3.
Fixes sonic-net#165

Author: vivekverma-arista

tests/qos/files/qos_params.th2.yaml

@@ -184,6 +184,34 @@ qos_params:
                 cell_size: 208
             hdrm_pool_wm_multiplier: 4
             cell_size: 208
+        topo-dualtor-aa-56:
+            50000_40m:
+                pcbb_xoff_1:
+                    dscp: 3
+                    ecn: 1
+                    pg: 3
+                    pkts_num_trig_pfc: 19939
+                    pkts_num_margin: 4
+                pcbb_xoff_2:
+                    dscp: 4
+                    ecn: 1
+                    pg: 4
+                    pkts_num_trig_pfc: 19939
+                    pkts_num_margin: 4
+                pcbb_xoff_3:
+                    outer_dscp: 2
+                    dscp: 3
+                    ecn: 1
+                    pg: 2
+                    pkts_num_trig_pfc: 19939
+                    pkts_num_margin: 4
+                pcbb_xoff_4:
+                    outer_dscp: 6
+                    dscp: 4
+                    ecn: 1
+                    pg: 6
+                    pkts_num_trig_pfc: 19939
+                    pkts_num_margin: 4
         topo-any:
             40000_300m:
                 pkts_num_leak_out: 0