hw/arm/aspeed_soc: Mark devices as user_creatable = false

QEMU currently aborts if the user is accidentially trying to
do something like this:

$ aarch64-softmmu/qemu-system-aarch64 -S -M integratorcp -nographic
QEMU 2.9.93 monitor - type 'help' for more information
(qemu) device_add ast2400
Unexpected error in error_set_from_qdev_prop_error()
 at hw/core/qdev-properties.c:1032:
Aborted (core dumped)

The ast2400 SoC devices are clearly not creatable by the user since
they are using the serial_hds and nd_table arrays directly in their
realize function, so mark them with user_creatable = false.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
