[CORE] Add mechanism to help irqchip override/update irq handler

Some irqchips (e.g. VGIC for irq routing) may require to override or
update the current irq handler set for given host irq number.

This patch extends the host irq APIs to allow get/set of host irq
handler and host irq handler data.

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