Optimize find_lock_granules for 2 granules

Currently the [find_lock_granules()](https://github.com/TF-RMM/tf-rmm/blob/main/lib/realm/src/granule.c#L186C13-L186C31) can handle multiple granules and lock them in the ascending order of addresses. But RMM uses these helpers only for 2 granules at the moment. The function can be optimized to handle just 2 granules.