hw/arm: Mark the "fsl,imx31" device with user_creatable = false

QEMU currently crashes when the user tries to instantiate the fsl,imx31
device manually:

$ aarch64-softmmu/qemu-system-aarch64 -M kzm -device fsl,,imx31
**
ERROR:/home/thuth/devel/qemu/tcg/tcg.c:538:tcg_register_thread:
 assertion failed: (n < max_cpus)
Aborted (core dumped)

The kzm board (which is the one that uses this CPU type) only supports
one CPU, and the realize function of the "fsl,imx31" device also uses
serial_hds[] directly, so this device clearly can not be instantiated
twice and thus we should mark it with user_creatable = false.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1509519537-6964-4-git-send-email-thuth@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
