Documentation: firmware-guide: gpio-properties: Fix factual mistakes

Fix factual mistakes and style issues in GPIO properties document.
This converts IoRestriction from InputOnly to OutputOnly as pins
in the example are used as outputs.

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