aspeed: Make the ast2600-a3 SoC not user creatable

Aspeed SoCs are complex devices that can not be specified on the
command line. Fix that to avoid QEMU aborts.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2227
Fixes: f25c0ae ("aspeed/soc: Add AST2600 support")
Reported-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-ID: <20240319150903.413662-1-clg@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

hw/arm/aspeed_ast2600.c

@@ -656,6 +656,8 @@ static void aspeed_soc_ast2600_class_init(ObjectClass *oc, void *data)
     AspeedSoCClass *sc = ASPEED_SOC_CLASS(oc);
 
     dc->realize      = aspeed_soc_ast2600_realize;
+    /* Reason: The Aspeed SoC can only be instantiated from a board */
+    dc->user_creatable = false;
 
     sc->name         = "ast2600-a3";
     sc->valid_cpu_types = valid_cpu_types;