irqchip/xilinx: Restructure and use jump label api

Add a global structure to house various variables.
And cleanup read/write handling by using jump label api.

Tested-by; Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Zubair Lutfullah Kakakhel <Zubair.Kakakhel@imgtec.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>