staging: greybus: gpio: switch GPIO portions to use GPIOLIB_IRQCHIP

Convert the GPIO driver to use the GPIO irqchip library
GPIOLIB_IRQCHIP instead of reimplementing the same.

Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Nishad Kamdar <nishadkamdar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>