leds: gpio: Use generic support for composing LED names

Switch to using generic LED support for composing LED class
device name.

Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Pavel Machek <pavel@ucw.cz>