irqchip/gic-v3-its: Make GICv4_ITS_LIST_MAX globally available

As we're about to make use of the maximum number of ITSs in
a GICv4 system, let's make this value global (and rename it to
GICv4_ITS_LIST_MAX).

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