AArch{32,64}: dynamically configure the number of GIC interrupts

In order to reduce the memory usage of large guests (as well
as improve performance), tell KVM about the number of interrupts
we require.

To avoid synchronization with the various device creation,
use a late_init callback to compute the GIC configuration.
[Andre: rename to gic__init_gic() to ease future expansion]

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>