irqchip/gic: Cleanup Franken-GIC handling

Introduce a static key identifying Samsung's unique creation, allowing
to replace the indirect call to compute the base addresses with
a simple test on the static key.

Faster, cheaper, negative diffstat.

Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>