sycl: Update gate logic for Alchemist GPUs regarding OneDNN features. (#26635)

* feat: updated gating logic of fattn-onednn.cpp

* verified device types

* Update ggml/src/ggml-sycl/fattn-onednn.cpp

Accepted recommendations to add bmg_g31 arch.

Co-authored-by: Neo Zhang <zhang.jianyu@outlook.com>

* Improved SPDA gate, added documentation.

* Added arch var to reworked gate, fixing build errors.

* Fix trailing whitespaces.

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Co-authored-by: Neo Zhang <zhang.jianyu@outlook.com>