libfdt: Change to FdtNodeMut::first_subnode(self) am: 973dda394a am: 492be929ff

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Virtualization/+/2964062

Change-Id: I95e58d34c5a71102545d80b2276edd1d560561fa
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>