google-sargo: stop HexagonRPCD on suspend (MR 5400)

There is a bug in FastRPC when waking from suspend. Since HexagonRPCD is
currently only useful for a few moments when the ADSP is requesting the
sensor registry, it can just be stopped without affecting sensor
support. Add a pre-suspend hook to stop HexagonRPCD so it doesn't crash
the ADSP when the device wakes up.

[ci:skip-build]: already built successfully in CI