irqchip/gic: Pass GIC pointer to save/restore functions

Instead of passing the GIC index to the save/restore functions pass a
pointer to the GIC chip data. This will allow these save/restore
functions to be re-used by a platform driver where the GIC chip data
structure is allocated dynamically and so there is no applicable index
for identifying the GIC.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>