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 <[email protected]>
Acked-by: Simon Horman <[email protected]>
Cc: Jason Cooper <[email protected]>
Link: http://lkml.kernel.org/r/1432891663-23641-1-git-send-email-geert%[email protected]
Signed-off-by: Thomas Gleixner <[email protected]>