docs: improve ARM passthrough doc

docs: improve ARM passthrough doc

Add a warning: use passthrough with care.

Add a pointer to the gic device tree bindings. Add an explanation on how
to calculate irq numbers from device tree.

Add a brief explanation of the reg property and a pointer to the xl docs
for a description of the iomem property. Add a note that in the example
we are using different memory addresses for guests and host.

Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Julien Grall <julien.grall@arm.com>