[nvidia-bluefield] Add default disk to platform json (#21250)

- Why I did it
On nvidia-bluefield, there is a eMMC along with the default NVMe disk. However, the ssdhealth command today picks up eMMC by default. Thus added this new field to platforn.json

Related to sonic-net/sonic-utilities#3693

- How I did it
Infra to read this is updated in the sonic-utilities show cli

- How to verify it
Verfied if show platform ssdhealth is reading the correct disk by default

device/nvidia-bluefield/arm64-nvda_bf-bf3comdpu/platform.json

@@ -6,7 +6,10 @@
         "fan_drawers": [],
         "psus": [],
         "thermals": [],
-        "sfps": []
+        "sfps": [],
+        "disk": {
+            "device" : "/dev/nvme0n1"
+        }
     },
     "interfaces": {
         "Ethernet0": {