Documentation: firmware-guide: gpio-properties: active_low only for GpioIo()

It appears that people may misinterpret active_low field in _DSD
for GpioInt() resource. Add a paragraph to clarify this.

Reported-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>