RISC-V: Convert cpu definition to future model

RISC-V: Convert cpu definition to future model

- Model borrowed from target/sh4/cpu.c
- Rewrote riscv_cpu_list to use object_class_get_list
- Dropped 'struct RISCVCPUInfo' and used TypeInfo array
- Replaced riscv_cpu_register_types with DEFINE_TYPES
- Marked base class as abstract
- Fixes -cpu list

Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Cc: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Michael Clark <mjc@sifive.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>