ACPI / documentation: Use recommended name in GPIO property names

The recommended property name for all kinds of GPIOs is to end it with
"-gpios" even if there is only one GPIO. Update the documentation to follow
this fact.

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