gpio: Add driver for Zynq GPIO controller

Add support for GPIO controller used by Xilinx Zynq.

v3 changes:
 - Use linux/gpio/driver.h instead of linux/gpio.h
 - Make irq a local variable in probe

v2 changes:
 - convert to pm_runtime_force_(suspend|resume)
 - add pm_runtime_set_active in probe()
 - also (un)prepare clocks when they are dis-/enabled
 - add some missing calls to pm_runtime_get()
 - use pm_runtime_put() instead of sync variant
 - remove gpio chip in driver remove()
 - remove redundant type casts
 - directly use IO helpers
 - use BIT macro to set/clear bits
 - migrate to GPIOLIB_IRQCHIP

Signed-off-by: Harini Katakam <harinik@xilinx.com>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>