gpiolib: acpi: change acpi_find_gpio() to accept firmware node

In preparation of switching all ACPI-based GPIO lookups to go through
acpi_find_gpio() let's change it to accept device node as its argument
as we do not always have access to device structure.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>