riscv, qemu_fw_cfg: Add support for RISC-V architecture

Qemu fw_cfg support was missing for RISC-V, which made it hard to do
proper vmcore dumps from qemu.

Add the missing RISC-V arch-defines.

You can now do vmcore dumps from qemu. Add "-device vmcoreinfo" to the
qemu command-line. From the qemu monitor:
  (qemu) dump-guest-memory vmcore

The vmcore can now be used, e.g., with the "crash" utility.

Acked-by: "Michael S. Tsirkin" <mst@redhat.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Tested-by: Clément Léger <cleger@rivosinc.com>
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Message-Id: <20231012102852.234442-1-bjorn@kernel.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>