target/riscv: Add a base 32 and 64 bit CPU

target/riscv: Add a base 32 and 64 bit CPU

At the same time deprecate the ISA string CPUs.

It is dobtful anyone specifies the CPUs, but we are keeping them for the
Spike machine (which is about to be depreated) so we may as well just
mark them as deprecated.

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