irqchip/gic-v3-its: Track per-ITS list number

At boot time, we enumerate all the GICv4-capable ITSs, and build
a mask of the available ITSs. Take this opportunity to store
the ITS number in the its_node structure so that we can use it
at a later time.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>