hw/riscv/opentitan: Correct OpenTitanState parent type/size

OpenTitanState is the 'machine' (or 'board') state: it isn't
a SysBus device, but inherits from the MachineState type.
Correct the instance size.
Doing so we  avoid leaking an OpenTitanState pointer in
opentitan_machine_init().

Fixes: fe0fe4735e ("riscv: Initial commit of OpenTitan machine")
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: <20230520054510.68822-6-philmd@linaro.org>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

include/hw/riscv/opentitan.h

@@ -55,10 +55,11 @@ struct LowRISCIbexSoCState {
 };
 
 #define TYPE_OPENTITAN_MACHINE MACHINE_TYPE_NAME("opentitan")
+OBJECT_DECLARE_SIMPLE_TYPE(OpenTitanState, OPENTITAN_MACHINE)
 
 typedef struct OpenTitanState {
     /*< private >*/
-    SysBusDevice parent_obj;
+    MachineState parent_obj;
 
     /*< public >*/
     LowRISCIbexSoCState soc;