irqchip: renesas-intc-irqpin: Document SoC-specific bindings

The documentation only mentioned the generic fallback compatible property.
Add the missing SoC-specific compatible properties, which are already in
use.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Link: https://lkml.kernel.org/r/1409212800-4957-1-git-send-email-geert+renesas@glider.be
Signed-off-by: Jason Cooper <jason@lakedaemon.net>