hw/arm/digic: Mark device with user_creatable = false

QEMU currently shows some unexpected behavior when the user trys to
do a "device_add digic" on an unrelated ARM machine like integratorcp
in "-nographic" mode (the device_add command does not immediately
return to the monitor prompt), and trying to "device_del" the device
later results in a "qemu/qdev-monitor.c:872:qdev_unplug: assertion
failed: (hotplug_ctrl)" error condition.
Looking at the realize function of the device, it uses serial_hds
directly and this means that the device can not be added a second
time, so let's simply mark it with "user_creatable = false" now.

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