riscv: spike: Add a generic spike machine

riscv: spike: Add a generic spike machine

Add a generic spike machine (not tied to a version) and deprecate the
spike mahines that are tied to a specific version. As we can now specify
the CPU via the command line we no londer need specific versions of the
spike machines.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Acked-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>