ARM: GICv3 ITS: initialize host ITS

ARM: GICv3 ITS: initialize host ITS

Map the registers frame for each host ITS and populate the host ITS
structure with some parameters describing the size of certain properties
like the number of bits for device IDs.

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