[Arista] Fix content of platform.json for DCS-720DT-48S (sonic-net#11855)

Why I did it
Content of platform.json was outdated and some platform_tests/api of sonic-mgmt were failing.

How I did it
Added the necessary values to platform.json

How to verify it
Running platform_tests/api of sonic-mgmt should yield 100% passrate.

Author: Staphylo

device/arista/x86_64-arista_720dt_48s/platform.json

@@ -2,15 +2,38 @@
     "chassis": {
         "name": "CCS-720DT-48S",
         "components": [],
-        "fans": [
-            {
-                "name": "fan1"
-            },
-            {
-                "name": "fan2"
+        "fan_drawers": [
+            {
+                "name": "fixed1",
+                "status_led": {
+                    "controllable": false,
+                    "available": false
+                },
+                "fans": [
+                    {
+                        "name": "fan1",
+                        "status_led": {
+                            "available": false
+                        }
+                    }
+                ]
+            },
+            {
+                "name": "fixed2",
+                "status_led": {
+                    "controllable": false,
+                    "available": false
+                },
+                "fans": [
+                    {
+                        "name": "fan2",
+                        "status_led": {
+                            "available": false
+                        }
+                    }
+                ]
             }
         ],
-        "fan_drawers": [],
         "psus": [
             {
                 "name": "psu1",
@@ -21,7 +44,10 @@
                 "temperature": false,
                 "voltage": false,
                 "voltage_high_threshold": false,
-                "voltage_low_threshold": false
+                "voltage_low_threshold": false,
+                "status_led": {
+                    "controllable": false
+                }
             },
             {
                 "name": "psu2",
@@ -32,21 +58,28 @@
                 "temperature": false,
                 "voltage": false,
                 "voltage_high_threshold": false,
-                "voltage_low_threshold": false
+                "voltage_low_threshold": false,
+                "status_led": {
+                    "controllable": false
+                }
             }
         ],
         "thermals": [
             {
-                "name": "Cpu temp sensor"
+                "name": "Cpu temp sensor",
+                "controllable": false
             },
             {
-                "name": "Psu temp sensor"
+                "name": "Psu temp sensor",
+                "controllable": false
             },
             {
-                "name": "SFP+ connector temp sensor"
+                "name": "SFP+ connector temp sensor",
+                "controllable": false
             },
             {
-                "name": "MAC external temp sensor"
+                "name": "MAC external temp sensor",
+                "controllable": false
             }
         ],
         "sfps": [