gpiolib: introduce fwnode_gpiod_get_index()

gpiolib: introduce fwnode_gpiod_get_index()

This introduces fwnode_gpiod_get_index() that iterates through common gpio
suffixes when trying to locate a GPIO within a given firmware node.

We also switch devm_fwnode_gpiod_get_index() to call
fwnode_gpiod_get_index() instead of iterating through GPIO suffixes on
its own.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Link: https://lore.kernel.org/r/20190913032240.50333-3-dmitry.torokhov@gmail.com
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>