of/irq: Document properties for wakeup interrupt parent

Some interrupt controllers in a SoC, are always powered on and have a
select interrupts routed to them, so that they can wakeup the SoC from
suspend. Add wakeup-parent DT property to refer to these interrupt
controllers.

Signed-off-by: Lina Iyer <ilina@codeaurora.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Link: https://lore.kernel.org/r/1573855915-9841-6-git-send-email-ilina@codeaurora.org