[mlir][aarch64] Disable bf16 tests on AArch64

This patch disables 2 bf16 tests that are currently not supported on
AArch64. I've triaged these failures and opened [1] to track this. I
don't have a simple reproducer for dense_output_bf16.mlir, but it's
rather clear that both tests fail due to missing support for `bfloat`
operations in the AArch64 backend.

I'm not sure what the path forward to enable these tests on AArch64
should be. I think that there are two options:
  * AArch64 backened gains capability to legalize these nodes containing
    `bfloat` operands, or
  * MLIR (similarly to Clang) is taught not to emit such nodes in the
    first place.

[1] https://github.com/llvm/llvm-project/issues/58465

Differential Revision: https://reviews.llvm.org/D136273