Define numbers for "nonstandard" versions about riscv-debug-spec

[mwachs5]

@asb did you have a more concrete proposal for this? Is this something done in the ISA which we can repeat the pattern?

[timsifive]

I can think of several variations of "nonstandard":

1. There is some extra functionality added. In that case it makes sense to report the version that you are compatible with, and have some other mechanism to discover/access the extra functionality.
2. Your debug implementation is so different that it doesn't make sense to use the spec. In that case, why do you even have those version numbers in that place?
3. You're using parts of the spec (eg. the JTAG transport to access a Debug Module) but not others (eg. the JTAG transport). In that case you might need a special version in the Debug Module indicating that even though the transport is compatible, the rest of the debug system is not.

Not super important, but if we want people to be able to pick and choose different layers in our debug solution, nice to have. Easiest way is to just spec it and be done. It's cheap.