target/riscv: Restructure deprecatd CPUs

target/riscv: Restructure deprecatd CPUs

Restructure the deprecated CPUs to make it clear in the code that these
are depreated. They are already marked as deprecated in
qemu-deprecated.texi. There are no functional changes.

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