ARM: GICv3 ITS: map ITS command buffer

ARM: GICv3 ITS: map ITS command buffer

Instead of directly manipulating the tables in memory, an ITS driver
sends commands via a ring buffer in normal system memory to the ITS h/w
to create or alter the LPI mappings.
Allocate memory for that buffer and tell the ITS about it to be able
to send ITS commands.

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