KVM: arm64: introduce ITS emulation file with stub functions

KVM: arm64: introduce ITS emulation file with stub functions

The ARM GICv3 ITS emulation code goes into a separate file, but
needs to be connected to the GICv3 emulation, of which it is an
option.
Introduce the skeleton with function stubs to be filled later.
Introduce the basic ITS data structure and initialize it, but don't
return any success yet, as we are not yet ready for the show.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Vadim Lomovtsev <Vadim.Lomovtsev@caviumnetworks.com>