docs/system/riscv: document acpi parameter of virt machine

docs/system/riscv: document acpi parameter of virt machine

Since QEMU v8.0.0 the RISC-V virt machine has a switch to disable ACPI
table generation. Add it to the documentation.

Fixes: 168b8c29cedb ("hw/riscv/virt: Add a switch to disable ACPI")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20231220193436.25909-1-heinrich.schuchardt@canonical.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>