armv7m: Delete unused "ARM,bitband-memory" devices

armv7m: Delete unused "ARM,bitband-memory" devices

These devices are optional, and enabled by property "enable-bitband".
armv7m_instance_init() creates them unconditionally, because the
property has not been set then.  armv7m_realize() realizes them only
when the property is true.  Works, although it leaves unrealized
devices hanging around in the QOM composition tree.  Affects machines
microbit, mps2-an505, mps2-an521, musca-a, and musca-b1.

Delete the unused devices by making armv7m_realize() unparent them.
Visible in "info qom-tree"; here's the change for microbit:

     /machine (microbit-machine)
       /microbit.twi (microbit.i2c)
         /microbit.twi[0] (qemu:memory-region)
       /nrf51 (nrf51-soc)
         /armv6m (armv7m)
           /armv7m-container[0] (qemu:memory-region)
    -      /bitband[0] (ARM,bitband-memory)
    -        /bitband[0] (qemu:memory-region)
    -      /bitband[1] (ARM,bitband-memory)
    -        /bitband[0] (qemu:memory-region)
           /cpu (cortex-m0-arm-cpu)

Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: qemu-arm@nongnu.org
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20200609122339.937862-7-armbru@redhat.com>