riscv: Move reg encoding helpers to kvm-cpu-arch.h

We'll need one of these helpers in the next patch in another file.
Let's proactively move them all now, since others may some day also
be useful.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20221018140854.69846-5-apatel@ventanamicro.com


Signed-off-by: Will Deacon <will@kernel.org>