ARM: dts: imx28-evk: Use pinctrl for gpio-led

Since commit 8fe4554f

 (leds: leds-gpio: adopt pinctrl support) gpio-led driver
has pinctrl support, so setup the gpio led pin via pinctrl and avoid the
following warning:

leds-gpio leds.12: pins are not configured from the driver

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>