[prim] Latch Enable signal for Sparse FSM macro

`prim_flop_macros.sv` defines `PRIM_FLOP_SPARSE_FSM()` macro. It makes the sparse fsm FFs simple. However, the `prim_sparse_fsm_flop` module does not have a latch enable signal, which is useful in certain cases.

This issue is to track the enhancement:

- Add `latch_en` port in `prim_sparse_fsm_flop` module
- Revise `PRIM_FLOP_SPARSE_FSM()` macro to have optional `__en` argument, (default tied to 1'b1).