hw/arm: Mark the "fsl,imx6" device with user_creatable = false

This device causes QEMU to abort if the user tries to instantiate it:

$ qemu-system-aarch64 -M sabrelite -smp 1,maxcpus=2 -device fsl,,imx6
Unexpected error in qemu_chr_fe_init() at chardev/char-fe.c:222:
qemu-system-aarch64: -device fsl,,imx6: Device 'serial0' is in use
Aborted (core dumped)

The device uses serial_hds[] directly in its realize function, so it
can not be instantiated again by the user.

Signed-off-by: Thomas Huth <thuth@redhat.com>
Message-id: 1509519537-6964-2-git-send-email-thuth@redhat.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
