hw/riscv/opentitan: Add TYPE_OPENTITAN_MACHINE definition

hw/riscv/opentitan: Add TYPE_OPENTITAN_MACHINE definition

QOM type names are usually defined as TYPE_FOO.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-Id: <20230520054510.68822-4-philmd@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>