gpio: add nexell driver

Changes in relation to FriendlyARM's U-Boot nanopi2-v2016.01:
- livetree API (dev_read_...) is used instead of fdt one (fdt...).

Signed-off-by: Stefan Bosch <stefan_b@posteo.net>
diff --git a/drivers/gpio/Makefile b/drivers/gpio/Makefile
index 7638259..334c5f2 100644
--- a/drivers/gpio/Makefile
+++ b/drivers/gpio/Makefile
@@ -64,4 +64,5 @@
 obj-$(CONFIG_PM8916_GPIO)	+= pm8916_gpio.o
 obj-$(CONFIG_MT7621_GPIO)	+= mt7621_gpio.o
 obj-$(CONFIG_MSCC_SGPIO)	+= mscc_sgpio.o
+obj-$(CONFIG_NX_GPIO)		+= nx_gpio.o
 obj-$(CONFIG_SIFIVE_GPIO)	+= sifive-gpio.o