ARM: GICv3: enable ITS on the host

ARM: GICv3: enable ITS on the host

Even though the ITS emulation is not yet in place, the host ITS already
gets initialized and Xen tries to map the host collections.
However for commands to be processed we need to *enable* the ITS, which
will be done in a later patch not yet merged.
So those MAPC commands are not processed and run into a timeout, leading
to a panic on machines which advertise an ITS in their DT.
This patch just enables the ITS (but not the LPIs on each redistributor),
to get those MAPC commands executed.

This fixes booting Xen on ARM64 machines with an ITS and the
(EXPERT) ITS Kconfig option enabled.

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