leds: ns2: Convert to GPIO descriptors

This converts the NS2 LED driver to use GPIO descriptors.
We take care to request the GPIOs "as is" which is what
the current driver goes to lengths to achieve, then we use
GPIOs throughout.

As the nodes for each LED does not have any corresponding
device, we need to use the DT-specific accessors to get these
GPIO descriptors from the device tree.

Cc: Vincent Donnefort <vdonnefort@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Tested-by: Simon Guinot <simon.guinot@sequanux.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>