[mlir][tosa] Add accumulator type attribute to TOSA dialect

Tosa supports fp16 and fp32 accumulator type for fp16 input, but
no way to tell for computational operators whether accumulator
should be fp16 or fp32 from input type. Add this new attribute
to specify the type.

Set to fp32 by default for now. When the time fp16 is supported,
the accumulator type can be selected based on trade-off between
performance and accuracy.

Reviewed By: jpienaar

Differential Revision: https://reviews.llvm.org/D146317