vexriscv: verilog: pull debug-enabled verilog

vexriscv: verilog: pull debug-enabled verilog

The upstream vexriscv repo now generates both the current VexRiscv.v
softcore, as well as VexRiscv-Debug.v.  This -Debug varient exposes
their specialized debug bus that allows for attaching a modified version
of openocd.

Sync the litex repo with the upstream version to take advantage of debug
support.

Signed-off-by: Sean Cross <sean@xobs.io>