gpio: X-Gene standby GPIO controller DTS binding

Update description for X-Gene standby GPIO controller DTS binding to
support GPIO line configuration as input, output or external IRQ pin.

Signed-off-by: Y Vo <yvo@apm.com>
Signed-off-by: Quan Nguyen <qnguyen@apm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>