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

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

Change-Id: Id8f429daf5ee9612e7f54120a23602ce62f9dcc4
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>