FROMGIT: 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>

Bug: 140053385
(cherry picked from commit 8594c3b8
 https://git.kernel.org/pub/scm/linux/kernel/git/maz/arm-platforms.git

 irq/ipi-as-irq)
Change-Id: I380d7bd5dac454ec7f9407512589e252c5504113
Signed-off-by: Eric Biggers <ebiggers@google.com>