ACPI / documentation: Fix ambiguity in the GPIO properties document

The first paragraph in Documentation/acpi/gpio-properties.txt is
ambiguous, so make it more clear.

Reported-by: Antonio Ospite <ao2@ao2.it>
Acked-by: Antonio Ospite <ao2@ao2.it>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>