[Mellanox] Platform update for DPU communication (sonic-net#20817)

- Why I did it
The dpu_tty.py cli tool requires some platform specific configuration for communication to DPU using picocom. 

- How I did it
The corresponding configuration is added to the platform.json to enable this tool for the smartswitch

- How to verify it
dpu_tty.py -n dpu0

device/mellanox/x86_64-nvidia_sn4280-r0/platform.json

@@ -653,6 +653,10 @@
             "interface": {
                 "Ethernet224": "Ethernet0"
             },
+            "serial-console": {
+                "device": "ttyUSB0",
+                "baud-rate": "115200"
+            },
             "rshim_info": "rshim0",
             "bus_info": "0000:08:00.0"
         },
@@ -661,6 +665,10 @@
             "interface": {
                 "Ethernet232": "Ethernet0"
             },
+            "serial-console": {
+                "device": "ttyUSB1",
+                "baud-rate": "115200"
+            },
             "rshim_info": "rshim1",
             "bus_info": "0000:07:00.0"
         },
@@ -669,6 +677,10 @@
             "interface": {
                 "Ethernet240": "Ethernet0"
             },
+            "serial-console": {
+                "device": "ttyUSB2",
+                "baud-rate": "115200"
+            },
             "rshim_info": "rshim2",
             "bus_info": "0000:01:00.0"
         },
@@ -677,6 +689,10 @@
             "interface": {
                 "Ethernet248": "Ethernet0"
             },
+            "serial-console": {
+                "device": "ttyUSB3",
+                "baud-rate": "115200"
+            },
             "rshim_info": "rshim3",
             "bus_info": "0000:02:00.0"
         }