aarch64: fdt: add PSCI compatible string tests
aarch64: fdt: add PSCI compatible string tests

Move the PSCI version to compatible code to a function so it can be more
easily tested and add a few basic unit tests.

BUG=b:227142928
TEST=tools/dev_container tools/run_tests --target=vm:aarch64

Change-Id: I383b6e9df76f26995adab6fe980fd29fe1fcdf0a
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosvm/+/3573362
Reviewed-by: Junichi Uekawa <6bd240a6ce32463e272bb69c685a8e946b33ac05@chromium.org>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Commit-Queue: Daniel Verkamp <72bc170b46ec491f7bdd4359a1c0bfed274de40c@chromium.org>

google/crosvm,google/crosvm,google/crosvm,google/crosvm