do you really need it? An FPGA is a digital device. What happens inside can be simulated with Vivado simulator, either behavioural or post-synthesis. No place for LTSpice or any other Spice flavour (no pun intended).
If what you want is to simulate the analog behaviour of I/Os and pins, the tool to use is the IBIS model (available from Xilinx) along with your PCB CAE tool (Altium, Cadence, Mentor...)