airoha: an7581: Adjust partition number after remove rootfs_data from kernel

Currenlty rootfs_data partition for overlay is mounted as loop device in
userspace. After revert two patches in kernel, number of partition has
changed. These reverts force to update partition number for 2nd bank to
be properly booted.

Revert of kernel patches:
https://dev.iopsys.eu/airoha/linux/-/merge_requests/148

airoha/an7581/base-files/etc/system_layout/emmc.json

@@ -9,7 +9,7 @@
     {
       "id": 2,
       "name": "B",
-      "rootfs": "/dev/mmcblk0p67",
+      "rootfs": "/dev/mmcblk0p66",
       "overlay": "/dev/mmcblk0p68"
     }
   ],