irqchip: renesas-irqc: Add DT binding documentation

The renesas-irqc driver supports DT, but the DT binding documentation was
missing.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Link: https://lkml.kernel.org/r/1409212800-4957-2-git-send-email-geert+renesas@glider.be


Signed-off-by: Jason Cooper <jason@lakedaemon.net>