UPSTREAM: DT/arm,gic-v3: Documment PPI partition support

Add a decription of the PPI partitioning support.

Change-Id: I11bb88c45556630207fb3ff534fa5645b73cb3f0
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: devicetree@vger.kernel.org
Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Will Deacon <will.deacon@arm.com>
Link: http://lkml.kernel.org/r/1460365075-7316-6-git-send-email-marc.zyngier@arm.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Caesar Wang <wxt@rock-chips.com>
(cherry picked from commit 287e9357abcc0ef079bf4e439e098a3bd6246a05)