ARM: vITS: handle MAPD command

ARM: vITS: handle MAPD command

The MAPD command maps a device by associating a memory region for
storing ITEs with a certain device ID. Since it features a valid bit,
MAPD also covers the "unmap" functionality, which we also cover here.
We store the given guest physical address in the device table, and, if
this command comes from Dom0, tell the host ITS driver about this new
mapping, so it can issue the corresponding host MAPD command and create
the required tables. We take care of rolling back actions should one
step fail.
Upon unmapping a device we make sure we clean up all associated
resources and release the memory again.
We use our existing guest memory access function to find the right ITT
entry and store the mapping there (in guest memory).

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