xen/arm: Allow device-passthrough even the IOMMU is off

xen/arm: Allow device-passthrough even the IOMMU is off

At the moment, we are only supporting device-passthrough when Xen has
enabled the IOMMU. There are some use cases where it is not possible to
use the IOMMU (e.g. doesn't exist, hardware limitation, performance) yet
it would be OK to assign a device to trusted domain so long they are
direct-mapped or the device doesn't do DMA.

Note that when the IOMMU is disabled, it will be necessary to add
xen,force-assign-without-iommu for every device that needs to be assigned.

Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Signed-off-by: Penny Zheng <penny.zheng@arm.com>
Tested-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Julien Grall <jgrall@amazon.com>