gpio: xgene: Fix kconfig for standby GIPO contoller

The standby GPIO controller can be used as a interrupt controller.
Select GPIOLIB_IRQCHIP when compiling this driver. Otherwise we get
a compilation error:

drivers/gpio/gpio-xgene-sb.c: In function 'xgene_gpio_sb_probe':
drivers/gpio/gpio-xgene-sb.c:312:10: error: 'struct gpio_chip'
  has no member named 'irqdomain'
  priv->gc.irqdomain = priv->irq_domain;
          ^
scripts/Makefile.build:295: recipe for target
'drivers/gpio/gpio-xgene-sb.o' failed
make[2]: *** [drivers/gpio/gpio-xgene-sb.o] Error 1

Fixes: 1013fc41 "gpio: xgene: Enable X-Gene standby GPIO as interrupt controller"
Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Acked-by: Quan Nguyen <qnguyen@apm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>