drivers/gpio: represent gpio-nomadik as an IRQ controller in DT documentation

Now gpio-nomadik has proper bindings devices initialised by Device Tree
entries are able to use the chained GPIO IRQ lines it provides. This
patch aims to reflect that in the gpio-nmk documentation.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>