irqchip/gic-v3-its: Fix misplaced __iomem annotations

Save 26 lines worth of Sparse complaints by fixing up this minor
mishap. The pointee lies in the __iomem space; the pointer does not.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>