devicetree: bindings: gpio-davinic: fix documentation

This patch adds missing #gpio-cells and also adds a
usage example for leds.

Reported-by: Alexander Holler <holler@ahsoftware.de>
Signed-off-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>