xen/arm: ffa: reclaim shared memory on guest destroy

When an FF-A enabled guest is destroyed it may leave behind memory
shared with SPs. This memory must be reclaimed before it's reused or an
SP may make changes to memory used by a new unrelated guest. So when the
domain is teared down add FF-A requests to reclaim all remaining shared
memory.

SPs in the secure world are notified using VM_DESTROYED that a guest has
been destroyed. An SP is supposed to relinquish all shared memory to allow
reclaiming the memory. The relinquish operation may need to be delayed if
the shared memory is for instance part of a DMA operation.

The domain reference counter is increased when the first FF-A shared
memory is registered and the counter is decreased again when the last
shared memory is reclaimed. If FF-A shared memory registrations remain
at the end of of ffa_domain_teardown() a timer is set to try to reclaim
the shared memory every second until the memory is reclaimed.

A few minor style fixes with a removed empty line here and an added new
line there.

Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>