irqchip: renesas: intc-irqpin: Improve binding documentation

irqchip: renesas: intc-irqpin: Improve binding documentation

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 705bc96c2c15313c ("irqchip:
    renesas-intc-irqpin: Add minimal runtime PM support")),
  - power-domains.

Add an example, taken from r8a7740.dtsi.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon 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.be
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>