sysbus: add a sysbus_mmio_unmap() helper

This will be used to remove the MMIO regions of the POWER9 XIVE
interrupt controller when the sPAPR machine is reseted.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20190513084245.25755-9-clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>