register: add vexriscv-specific registers

VexRiscv contains its own custom registers for external interrupt
mapping.  Add these registers under the `register::vexriscv` namespace,
and prefix the assembly routines with "v-" in order to avoid ambiguity
with e.g. `sip` and `mip` registers.

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