soc: vexriscv: add cpu debug support
Add support for debugging the CPU, and gate it behind a new cpu_debug
parameter. With this enabled, a simple Wishbone interface is provided.
The debug version of the core adds two 32-bit registers to the CPU.
The register at address 0 indicates status, and is used to halt
and reset the core.
The debug register at address 4 is used to inject opcodes into the
core, and read back the result.
A patched version of OpenOCD can be used to attach to this bus via
the Litex Ethernet or UART bridges.
Signed-off-by: Sean Cross <sean@xobs.io>