Documentation: firmware-guide: gpio-properties: Clarify initial output state

GpioIo() doesn't provide an explicit state for an output pin.
Linux tries to be smart and uses a common sense based on other
parameters. Document how it looks like in the code.

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