Clocked DSP in yosys+nextpnr toolchain

[suarezvictor]

In current state, nextpnr-xilinx reports this error when trying to use clocked DSPs: ERROR: Clocked DSP48E1s are currently unsupported

The way to mitigate it is by adding this command to yosys: scratchpad -set xilinx_dsp.multonly 1

The patch can be like adding self._yosys_cmds += ["scratchpad -set xilinx_dsp.multonly 1"] here: https://github.com/enjoy-digital/litex/blob/master/litex/build/xilinx/yosys_nextpnr.py#L48