ARM: GICv3 ITS: parse and store ITS subnodes from hardware DT

ARM: GICv3 ITS: parse and store ITS subnodes from hardware DT

Parse the GIC subnodes in the device tree to find every ITS MSI controller
the hardware offers. Store that information in a list to both propagate
all of them later to Dom0, but also to be able to iterate over all ITSes.
This introduces an ITS Kconfig option (as an EXPERT option), use
XEN_CONFIG_EXPERT=y on the make command line to see and use the option.

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