target/riscv/cpu.c: restrict 'mimpid' value

target/riscv/cpu.c: restrict 'mimpid' value

Following the same logic used with 'mvendorid' let's also restrict
'mimpid' for named CPUs. Generic CPUs keep setting the value freely.

Note that we're getting rid of the default RISCV_CPU_MARCHID value. The
reason is that this is not a good default since it's dynamic, changing
with with every QEMU version, regardless of whether the actual
implementation of the CPU changed from one QEMU version to the other.
Named CPU should set it to a meaningful value instead and generic CPUs
can set whatever they want.

This is the error thrown for an invalid 'mimpid' value for the veyron-v1
CPU:

$ ./qemu-system-riscv64 -M virt -nographic -cpu veyron-v1,mimpid=2
qemu-system-riscv64: can't apply global veyron-v1-riscv-cpu.mimpid=2:
    Unable to change veyron-v1-riscv-cpu mimpid (0x111)

Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230706101738.460804-5-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>