gpio-sysfs: Use gpio descriptor name instead of gpiochip names array

The name is now stored in the gpio descriptor as well, for example to
allow to store names from DT. This patch changes the sysfs gpio files
to use the gpio descriptor name.

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