leds: netxbig: Convert to use GPIO descriptors

This converts the NetXbig LED driver to use GPIO descriptors
instead of using the legacy interfaces in <linux/of_gpio.h>
and <linux/gpio.h> to iteratively parse the device tree for
global GPIO numbers.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Tested-by: Simon Guinot <simon.guinot@sequanux.org>