gpio: gpio-xilinx: Add interrupt support

gpio: gpio-xilinx: Add interrupt support

Adds interrupt support to the Xilinx GPIO driver so that rising and
falling edge line events can be supported. Since interrupt support is
an optional feature in the Xilinx IP, the driver continues to support
devices which have no interrupt provided.
Depends on OF_GPIO framework for of_xlate function to translate
gpiospec to the GPIO number and flags.

Signed-off-by: Robert Hancock <hancock@sedsystems.ca>
Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Signed-off-by: Srinivas Neeli <srinivas.neeli@xilinx.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>