gpiolib: introduce devm_fwnode_gpiod_get_index()

devm_fwnode_get_index_gpiod_from_child() is too long, besides the fwnode
in question does not have to be a child of device node. Let's rename it
to devm_fwnode_gpiod_get_index() and keep the old name for compatibility
for now.

Also let's add a devm_fwnode_gpiod_get() wrapper as majority of the
callers need a single GPIO.

Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Link: https://lore.kernel.org/r/20190913032240.50333-2-dmitry.torokhov@gmail.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>