ARM: S3C24XX: Free the backlight gpio requested in Mini2440 board code

The backlight gpio must not be left requested in the board
code, otherwise s3c24xx_led driver can't successfully claim it.
So request the backlight gpio, configure it to proper state and
gpio_free right away.

This change is required for converting the s3c24xx_led driver
to the gpiolib API.

Signed-off-by: Sylwester Nawrocki <sylvester.nawrocki@gmail.com>
Acked-by: Juergen Beisert <jbe@pengutronix.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>