gpio: Fix ngpio in gpio-xilinx driver

If one adds gpio-controller; to the chip in the devicetree, then
initialization fails with 'gpiochip_find_base: cannot find free range',
because ngpio is 0. This patch fixes the bug.

This version includes the suggestions from Linus Walleij.

Tested on ml507 board.

Signed-off-by: Gernot Vormayr <gvormayr@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>