gpio-omap.txt: add reg and interrupts properties

Document properties reg and interrupts for OMAP GPIO controller bindings

Also add unit address in node name of the example

Signed-off-by: Andreas Klinger <ak@it-klinger.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Rob Herring <robh@kernel.org>