When removing a mapping from a domain, we need to send an invalidation to
all devices that might have stored it in their Address Translation Cache
(ATC). In addition when updating the context descriptor of a live domain,
we'll need to send invalidations for all devices attached to it.

Maintain a list of devices in each domain, protected by a spinlock. It is
updated every time we attach or detach devices to and from domains.

It needs to be a spinlock because we'll invalidate ATC entries from
within hardirq-safe contexts, but it may be possible to relax the read
side with RCU later.
Signed-off-by: Jean-Philippe Brucker <jean-philippe.brucker@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>