[ARM] vgic: Increase max IRQ per-guest to 256

Quite a few guest including VExpress-A9 and VExpress-A15 have more
than 128 interrupts so far most of them were not used but latest
linux kernel uses interrupts beyond 128 hence this patch increases
max IRQ per-guest for VGIC to 256.

Signed-off-by: Anup Patel <anup@brainfault.org>