irqchip: GICv3: Check if GIC register access is controlled

Add support to configure ITS registers only if higher
exception levels have not already configured them.

Change-Id: I45eaa51e56e034d011cf41d8b924fb674f63447d
Signed-off-by: Hanumant Singh <hanumant@codeaurora.org>
Signed-off-by: Puja Gupta <pujag@codeaurora.org>