fpga: Add a xilinx_specific fileset to microwatt.core

fpga: Add a xilinx_specific fileset to microwatt.core

At present this just has the Xilinx-specific multiplier code, but
might in future have other things.

This also adds the xilinx_specific fileset to the synth target.
Without that it was failing because there was no multiplier.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>