libfdt: Make Fdt::root{,_mut}() infallible

As we now build the node without calling into libfdt, there's no need
for the Rust functions to -> Result<_> so update the API accordingly.

Test: m pvmfw virtmgr fsfdt librialto libfdtpci
Test: liblibfdt.integration_test libpvmfw.device_assignment.test
Change-Id: I82d146b0db851c72cb3d58516f4cc14fff3f0321
diff --git a/pvmfw/src/device_assignment.rs b/pvmfw/src/device_assignment.rs
index 54b5a47..e427710 100644
--- a/pvmfw/src/device_assignment.rs
+++ b/pvmfw/src/device_assignment.rs
@@ -715,7 +715,7 @@
     }
 
     fn patch_pviommus(&self, fdt: &mut Fdt) -> Result<BTreeMap<PvIommu, Phandle>> {
-        let mut compatible = fdt.root_mut()?.next_compatible(Self::PVIOMMU_COMPATIBLE)?;
+        let mut compatible = fdt.root_mut().next_compatible(Self::PVIOMMU_COMPATIBLE)?;
         let mut pviommu_phandles = BTreeMap::new();
 
         for pviommu in &self.pviommus {