irqchip/gic: Document optional Clock and Power Domain properties

Depending on the GIC variant, the GIC module has one or more clock
inputs. Document the optional "clocks" and "clock-names" properties,
and their possible values, based on the Technical Reference Manuals.
optional.

Add the optional "power-domains" property.

This will allow to describe in DT the relationship between the GIC and
the Clock and/or Power Domain topology on SoCs where this is relevant
and needed for proper operation.

Note: As the current GIC driver doesn't support Runtime PM yet, PM
Domain constraints must be handled elsewhere in e.g. platform code.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh+dt@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Kumar Gala <galak@codeaurora.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Marc Zyngier <marc.zyngier@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Link: http://lkml.kernel.org/r/1442261204-30931-3-git-send-email-geert%2Brenesas@glider.be


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>