irqchip/gic-v3: Claim iomem resources

As a simple quality-of-life tweak, claim our MMIO regions when mapping
them, such that the GIC shows up in /proc/iomem. No effort is spent on
trying to release them, since frankly if the GIC fails to probe then
it's never getting a second try anyway.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/c534c2a458a3bf94ccdae8abc6edc3d45a689c30.1649777295.git.robin.murphy@arm.com