gpio: add Xilinx Zynq PS GPIO driver

Most of the code is taken (and adapted) from Linux kernel driver.

Just add CONFIG_ZYNQ_GPIO to you config to enable it

Signed-off-by: Andrea Scian <andrea.scian@dave.eu>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/drivers/gpio/Makefile b/drivers/gpio/Makefile
index 8ca8b05..fb40e09 100644
--- a/drivers/gpio/Makefile
+++ b/drivers/gpio/Makefile
@@ -43,3 +43,4 @@
 obj-$(CONFIG_SUNXI_GPIO)	+= sunxi_gpio.o
 obj-$(CONFIG_LPC32XX_GPIO)	+= lpc32xx_gpio.o
 obj-$(CONFIG_STM32_GPIO)	+= stm32_gpio.o
+obj-$(CONFIG_ZYNQ_GPIO)		+= zynq_gpio.o