vexriscv: put debug bus directly on wishbone bus

vexriscv: put debug bus directly on wishbone bus

By placing the VexRiscv debug bus on the Wishbone bus, the Etherbone
core can access 32-bit values directly from the core.  Additionally,
both reading and writing are supported without the need to do a SYNC
register as before.

Additionally, the address of the Wishbone bus won't move around anymore,
as it's fixed when doing `self.register_mem()`.

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