target/riscv: add CPU QOM header

target/riscv: add CPU QOM header

QMP CPU commands are usually implemented by a separated file,
<arch>-qmp-cmds.c, to allow them to be build only for softmmu targets.
This file uses a CPU QOM header with basic QOM declarations for the
arch.

We'll introduce query-cpu-definitions for RISC-V CPUs in the next patch,
but first we need a cpu-qom.h header with the definitions of
TYPE_RISCV_CPU and RISCVCPUClass declarations. These were moved from
cpu.h to the new file, and cpu.h now includes "cpu-qom.h".

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230411183511.189632-2-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>