Add missing documentation for required properties:
  - interrupt-controller,
  - parent interrupts (one entry per provided interrupt).

Add missing documentation for optional properties:
  - functional clock (managed since commit 705bc96c ("irqchip:
    renesas-intc-irqpin: Add minimal runtime PM support")),
  - power-domains.

Add an example, taken from r8a7740.dtsi.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Acked-by: NSimon Horman <horms+renesas@verge.net.au>
Cc: Jason Cooper <jason@lakedaemon.net>
Link: http://lkml.kernel.org/r/1432891663-23641-1-git-send-email-geert%2Brenesas@glider.beSigned-off-by: NThomas Gleixner <tglx@linutronix.de>