Fix a test to check duplicated phandles

All the dtbo will be merged into one fdt to verify if dtb in a test
device is the same as expected.
However, there are cases the number of phandle will be
duplicated
Fix a test to get correct phandle_offset.

Bug: 259891464
Test: run vts -m VtsFirmwareDtboVerification
Change-Id: Ia90a4537a4be0dfbb0316d2b47fb12d619fa7c34
(cherry picked from commit 7bcbff31c5af60933701281d5e682e62732143d8)