write_json: dump default parameter values
write_json: dump default parameter values

Fixes #1823.

This will allow nextpnr to reuse the default value information already
present in yosys cells_sim.v and avoid duplicating (and probably
desyncing) this information.

cliffordwolf/yosys,SymbiFlow/yosys,antmicro/yosys,YosysHQ/yosys,YosysHQ/yosys,cliffordwolf/yosys,YosysHQ/yosys,YosysHQ/yosys,SymbiFlow/yosys,antmicro/yosys,cliffordwolf/yosys,cliffordwolf/yosys,antmicro/yosys,cliffordwolf/yosys,SymbiFlow/yosys,cliffordwolf/yosys,antmicro/yosys,cliffordwolf/yosys,antmicro/yosys,SymbiFlow/yosys,antmicro/yosys,SymbiFlow/yosys,SymbiFlow/yosys,YosysHQ/yosys,antmicro/yosys,YosysHQ/yosys,SymbiFlow/yosys,antmicro/yosys,YosysHQ/yosys,cliffordwolf/yosys,cliffordwolf/yosys,YosysHQ/yosys,SymbiFlow/yosys