gpiolib: Use GPIO name from names array for gpio descriptor

This patch adds GPIO names to the GPIO descriptors when initializing the
gpiochip. It also introduces a check whether any of the new names will
conflict with an existing GPIO name.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>