DSP disable option for yosys+nextpnr toolchain

[suarezvictor]

When yosys is called with DSP enabled for xilinx target, nextpnr-xilinx gives this error: ERROR: Clocked DSP48E1s are currently unsupported

Solution: add -nodsp option in https://github.com/enjoy-digital/litex/blob/master/litex/build/xilinx/yosys_nextpnr.py#L166 'yosys -p "synth_xilinx -flatten -abc9 -nobram -nodsp -arch xc7 -top $(TOP); write_json $(TOP).json" $(VERILOG) > /dev/null'

(ideally those options should be seteable in class YosysNextpnrToolchain)

test image after multiplier-in-fabric solution: