[mlir] Fix generation of AVX512 dialect documentation

This changes adjusts the documentation generation for the AVX512 dialect. The machanism to generate documentation was changed with https://github.com/llvm/llvm-project/commit/1a083f027f33f4014247df4c0e757e23d5cdab64.

Reviewed By: nicolasvasilache

Differential Revision: https://reviews.llvm.org/D87460