irqchip/gic-v3-its: Split allocation from initialisation of its_node

In order to pave the way for more fancy quirk handling without making
more of a mess of this terrible driver, split the allocation of the
ITS descriptor (its_node) from the actual probing.

This will allow firmware-specific hooks to be added between these
two points.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20231006125929.48591-4-lpieralisi@kernel.org