target/riscv/kvm: add scounteren CSR

target/riscv/kvm: add scounteren CSR

Add support for the scounteren KVM CSR. Note that env->scounteren is a
32 bit and all KVM CSRs are target_ulong, so scounteren will be capped
to 32 bits read/writes.

Reported-by: Andrew Jones <ajones@ventanamicro.com>
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: <20250429124421.223883-10-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>