ACPI / gpio: Split acpi_get_gpiod_by_index()

Split acpi_get_gpiod_by_index() into three smaller routines to
allow the subsequent change of the generic firmware node properties
code to be more strarightforward.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>