[Tosa] Match the reference TFL Exp calculation in the Softmax TOSA legalization
Hi,

This PR modifies the LUT calculation for the EXP part of the `tfl.softmax` TOSA legalization to match the [TFL Softmax reference kernel](https://github.com/tensorflow/tensorflow/blob/5d4c1b0c67e2cfdcdea416e2314fa676aa072c07/tensorflow/lite/kernels/internal/reference/softmax.h#L65) for a bit-exact result.