hw/ppc/spapr_rtc: Mark the RTC device with user_creatable = false

QEMU currently aborts unexpectedly when a user tries to do something
like this:

$ qemu-system-ppc64 -nographic -S -nodefaults -monitor stdio
QEMU 2.9.92 monitor - type 'help' for more information
(qemu) device_add spapr-rtc,id=spapr-rtc
(qemu) device_del spapr-rtc
**
ERROR:qemu/qdev-monitor.c:872:qdev_unplug: assertion failed: (hotplug_ctrl)
Aborted (core dumped)

The RTC device is not meant to be hot-pluggable - it's an internal
device only and it even should not be possible to create it a
second time with the "-device" parameter, so let's mark this
with "user_creatable = false".

Signed-off-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
