iommu/arm: Add ability to handle deferred probing request

iommu/arm: Add ability to handle deferred probing request

This patch adds minimal required support to General IOMMU framework
to be able to handle a case when IOMMU driver requesting deferred
probing for a device.

In order not to pull Linux's error code (-EPROBE_DEFER) to Xen
we have chosen -EAGAIN to be used for indicating that device
probing is deferred.

This is needed for the upcoming IPMMU driver which may request
deferred probing depending on what device will be probed the first
(there is some dependency between these devices, Root device must be
registered before Cache devices. If not the case, driver will deny
further Cache device probes until Root device is registered).
As we can't guarantee a fixed pre-defined order for the device nodes
in DT, we need to be ready for the situation where devices being
probed in "any" order.

Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@epam.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>