[mlir][tosa] Update tosa.avg_pool2d for bit-exact TOSA behavior

The normalization component of average pool has a very specific
rounding behavior for compouting the division for floating
point values. Updated so that the bit-exact version is implemented.

Also includes a fix for computing the stride part of the average pool
operation.

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D141339