irqchip/gic: Don't initialise chip if mapping IO space fails

If we fail to map the address space for the GIC distributor or CPU
interface, then don't attempt to initialise the chip, just WARN and
return.

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>