[SYCL] Modernize prefetch property_key creation (#15904)
Simplified key creation was introduced in
https://github.com/intel/llvm/pull/12831 but this one was never updated.
I think it happened because this property never defined `PropKind` enum
entry and I'm not even sure how it worked or what are scenarios where it
was broken.
Regardless, fix and unify with all other properties with this PR.
