[TOSA] Add legalize stateful pass to TOSA
Handle the TFL Variable operators in a separate module level pass
since the existing tfl-to-tosa pass is a function pass.

TFL::VarHandleOp is mapped to TOSA::VariableOp
TFL::AssignVariableOp is mapped to TOSA::VariableWriteOp
TFL::ReadVariableOp is mapped to TOSA::VariableReadOp