greybus: gpio: add interrupt handling support

Adds gpio interrupt handling support using an irqchip/irqdomain
instantiation inside the GB GPIO driver. This implementation works
on older kernels such as 3.10 that do not have the gpiolib irqchip
helpers. Any line on a Greybus gpiochip may be configured as an
interrupt. Once configured, IRQ event messages received from a
module fire off the registered interrupt handler.
Signed-off-by: Matt Porter <mporter@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>