hw/riscv/opentitan: Explicit machine type definition

hw/riscv/opentitan: Explicit machine type definition

Expand the DEFINE_MACHINE() macro, converting the class_init()
handler.

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: <20230520054510.68822-5-philmd@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>