[mlir][Tosa] fix fp16/bf16 support for AvgPool2d (#68718)

Currently, the AvgPool2d operation in the TOSA MLIR dialect does not
accept half-precision Fp16 and Bf16 tensors, conversely to what stated
in the [TOSA
specification](https://www.mlplatform.org/tosa/tosa_spec.html#_avg_pool2d).
This issue was previously raised: #63424 here on Github and it is due to
a bug in the AvgPool2d verifier.

This patch fixes the AvgPool2d verifier to accept fp16 & bf16 datatype
for input/output tensors and accumulator, and it adds related LIT test
cases in Tosa/ops.mlir.