[fdt] Add function to get the first range in /memory node of fdt

and test it. This function will be reused later in both pvmfw and
rialto.

Test: atest liblibfdt.integration_test
Test: atest vmbase_example.integration_test
Test: m pvmfw_img
Bug: 284462758
Change-Id: Ic547530f911281c2db14b9a59b7e2470be10361f
diff --git a/vmbase/example/src/main.rs b/vmbase/example/src/main.rs
index 90a0ec2..adda406 100644
--- a/vmbase/example/src/main.rs
+++ b/vmbase/example/src/main.rs
@@ -195,7 +195,7 @@
 }
 
 fn check_fdt(reader: &Fdt) {
-    for reg in reader.memory().unwrap().unwrap() {
+    for reg in reader.memory().unwrap() {
         info!("memory @ {reg:#x?}");
     }