gpiolib: Introduce gpiochip_node_count() helper

The gpiochip_node_count() helper iterates over the device child nodes that
have the "gpio-controller" property set. It returns the number of such nodes
under a given device.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Bartosz Golaszewski <brgl@bgdev.pl>