hw/intc/arm_gicv3: Add accessors for ICH_ system registers

hw/intc/arm_gicv3: Add accessors for ICH_ system registers

The GICv3 virtualization interface includes system registers
accessible only to the hypervisor which form the control
interface for interrupt virtualization. Implement these
registers.

The function gicv3_cpuif_virt_update() which determines
whether it needs to signal vIRQ, vFIQ or a maintenance
interrupt is introduced here as a stub function -- its
implementation will be added in a subsequent commit.

Signed-off-by: Peter Maydell <[email protected]>
Message-id: 1483977924[email protected]