This adds a simple bus that can be mastered from an external
system via JTAG, which will be used to hookup various debug
It's loosely based on the RiscV model (hence the DMI name).
The module currently only supports hooking up to a Xilinx BSCANE2
but it shouldn't be too hard to adapt it to support different TAPs
The JTAG protocol proper is not exactly the RiscV one at this point,
though I might still change it.
This comes with some sim variants of Xilinx BSCANE2 and BUFG and a
Signed-off-by: Benjamin Herrenschmidt <firstname.lastname@example.org>