ARM: GICv3 ITS: introduce ITS command handling

ARM: GICv3 ITS: introduce ITS command handling

To be able to easily send commands to the ITS, create the respective
wrapper functions, which take care of the ring buffer.
The first two commands we implement provide methods to map a collection
to a redistributor (aka host core) and to flush the command queue (SYNC).
Start using these commands for mapping one collection to each host CPU.
As an ITS might choose between *two* ways of addressing a redistributor,
we store both the MMIO base address as well as the processor number in
a per-CPU variable to give each ITS what it wants.

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