spapr_vio/spapr_iommu: Move VIO bypass where it belongs

Instead of tweaking a TCE table device by adding there a bypass flag,
let's add an alias to RAM and IOMMU memory region, and enable/disable
those according to the selected bypass mode.
This way IOMMU memory region can have size of the actual window rather
than ram_size which is essential for upcoming DDW support.

This moves bypass logic to VIO layer and keeps @bypass flag in TCE table
for migration compatibility only. This replaces spapr_tce_set_bypass()
calls with explicit assignment to avoid confusion as the function could
do something more that just syncing the @bypass flag.

This adds a pointer to VIO device into the sPAPRTCETable struct to provide
the sPAPRTCETable device a way to update bypass mode for the VIO device.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
