[MLIR][GPU] Make max flat work group size for ROCDL kernels configurable

While the default value for the amdgpu-flat-work-group-size attribute,
"1, 256", matches the defaults from Clang, some users of the ROCDL dialect,
namely Tensorflow, use larger workgroups, such as 1024. Therefore,
instead of hardcoding this value, we add a rocdl.max_flat_work_group_size
attribute that can be set on GPU kernels to override the default value.

Reviewed By: whchung

Differential Revision: https://reviews.llvm.org/D115741